>>>Exactly - I was going to tell that it's not going to work this way because of multiple inserts possibility.
>>>
>>>But would this work?
>>>

>>>    INSERT INTO ProductFileData
>>>    (ProductFileColumnId,ProductFileRowId,ProductFileId)
>>>    SELECT INSERTED.Id,NewID(),INSERTED.ProductFileId
>>>    FROM INSERTED INNER JOIN ProductFileData ON ProductFileData.ProductFileId = INSERTED.ProductFileId

>>>
>>>Or why not let ProductFileRowID field to have default NewID() and then you don't need to include it in insert?
>>
>>Actually what I posted above is complete rubbish. The ProductFileRowId has to be based on existing unique values for existing rows.
>>Using NEWID() is completely wrong. Using a default would be worse because all the rows would have different Ids. Let me try again....
>
>Can you post the problem definition again with the table creation scripts and some data?

OK:

USE [LLNET4]
GO
/****** Object:  User [LLNETUSER]    Script Date: 07/08/2009 18:06:22 ******/
CREATE USER [LLNETUSER] FOR LOGIN [LLNETUSER] WITH DEFAULT_SCHEMA=[dbo]
GO
/****** Object:  Table [dbo].[Owners]    Script Date: 07/08/2009 18:06:22 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[Owners](
	[Id] [uniqueidentifier] ROWGUIDCOL  NOT NULL CONSTRAINT [DF_Owners_Id]  DEFAULT (newid()),
	[Name] [nvarchar](50) NULL,
	[Password] [nvarchar](50) NULL,
 CONSTRAINT [PK_Owners] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object:  Table [dbo].[ProductFileData]    Script Date: 07/08/2009 18:06:22 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[ProductFileData](
	[Id] [uniqueidentifier] ROWGUIDCOL  NOT NULL CONSTRAINT [DF_ProductFileData_Id]  DEFAULT (newid()),
	[ProductFileColumnId] [uniqueidentifier] NULL,
	[ProductFileRowId] [uniqueidentifier] NULL,
	[ProductFileId] [uniqueidentifier] NULL,
	[ObjectData] [varbinary](max) NULL,
	[TextData] [nvarchar](max) NULL,
 CONSTRAINT [PK_ProductFileData] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
/****** Object:  Table [dbo].[VisualObjects]    Script Date: 07/08/2009 18:06:22 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[VisualObjects](
	[Id] [uniqueidentifier] ROWGUIDCOL  NOT NULL CONSTRAINT [DF_VisualObjects_Id]  DEFAULT (newid()),
	[OwnerId] [uniqueidentifier] NULL,
	[ProductFileColumnId] [uniqueidentifier] NULL,
	[Name] [nvarchar](50) NULL,
	[Object] [varbinary](max) NULL,
	[ObjectType] [int] NULL,
 CONSTRAINT [PK_VisualObjects] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
/****** Object:  Table [dbo].[ProductFiles]    Script Date: 07/08/2009 18:06:22 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[ProductFiles](
	[Id] [uniqueidentifier] ROWGUIDCOL  NOT NULL CONSTRAINT [DF_ProductFiles_Id]  DEFAULT (newid()),
	[OwnerId] [uniqueidentifier] NULL,
	[Name] [nvarchar](50) NULL,
 CONSTRAINT [PK_ProductFiles] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object:  Table [dbo].[ProductFileColumns]    Script Date: 07/08/2009 18:06:22 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[ProductFileColumns](
	[Id] [uniqueidentifier] ROWGUIDCOL  NOT NULL CONSTRAINT [DF_ProductFileColumns_Id]  DEFAULT (newid()),
	[ProductFileId] [uniqueidentifier] NULL,
	[Name] [nvarchar](50) NULL,
 CONSTRAINT [PK_ProductFileColumns] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object:  Trigger [AddProductDataRows]    Script Date: 07/08/2009 18:06:22 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author:		Viv Phillips
-- Create date: 08/07/2009
-- Description:	Add data records for each existing row and the added column(s)
-- =============================================
CREATE TRIGGER [dbo].[AddProductDataRows]
   ON  [dbo].[ProductFileColumns]
   AFTER INSERT
AS 
BEGIN
    INSERT INTO ProductFileData
    (ProductFileColumnId,ProductFileRowId,ProductFileId)
    SELECT DISTINCT INSERTED.Id,ProductFileData.ProductFileRowId,INSERTED.ProductFileId 
    FROM INSERTED INNER JOIN ProductFileData ON ProductFileData.ProductFileId = INSERTED.ProductFileId
END
GO
/****** Object:  StoredProcedure [dbo].[AddRow]    Script Date: 07/08/2009 18:06:22 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author:		<Author,,Name>
-- Create date: <Create Date,,>
-- Description:	<Description,,>
-- =============================================
CREATE PROCEDURE [dbo].[AddRow]
    @ProductFileId uniqueidentifier
AS
BEGIN
    DECLARE @newid uniqueidentifier
    SET @newid = NEWID()
    INSERT INTO ProductFileData
    (ProductFileId,ProductFileColumnId,ProductFileRowId)
     SELECT @ProductFileId,ProductFileColumns.Id,@newid FROM ProductFileColumns WHERE ProductFileColumns.ProductFileId = @ProductFileId
END
GO
/****** Object:  ForeignKey [FK_ProductFileColumns_ProductFiles]    Script Date: 07/08/2009 18:06:22 ******/
ALTER TABLE [dbo].[ProductFileColumns]  WITH CHECK ADD  CONSTRAINT [FK_ProductFileColumns_ProductFiles] FOREIGN KEY([ProductFileId])
REFERENCES [dbo].[ProductFiles] ([Id])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[ProductFileColumns] CHECK CONSTRAINT [FK_ProductFileColumns_ProductFiles]
GO
/****** Object:  ForeignKey [FK_ProductFileData_ProductFileColumns]    Script Date: 07/08/2009 18:06:22 ******/
ALTER TABLE [dbo].[ProductFileData]  WITH CHECK ADD  CONSTRAINT [FK_ProductFileData_ProductFileColumns] FOREIGN KEY([ProductFileColumnId])
REFERENCES [dbo].[ProductFileColumns] ([Id])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[ProductFileData] CHECK CONSTRAINT [FK_ProductFileData_ProductFileColumns]
GO
/****** Object:  ForeignKey [FK_ProductFiles_Owners]    Script Date: 07/08/2009 18:06:22 ******/
ALTER TABLE [dbo].[ProductFiles]  WITH CHECK ADD  CONSTRAINT [FK_ProductFiles_Owners] FOREIGN KEY([OwnerId])
REFERENCES [dbo].[Owners] ([Id])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[ProductFiles] CHECK CONSTRAINT [FK_ProductFiles_Owners]
GO
/****** Object:  ForeignKey [FK_VisualObjects_ProductFileColumns]    Script Date: 07/08/2009 18:06:22 ******/
ALTER TABLE [dbo].[VisualObjects]  WITH CHECK ADD  CONSTRAINT [FK_VisualObjects_ProductFileColumns] FOREIGN KEY([ProductFileColumnId])
REFERENCES [dbo].[ProductFileColumns] ([Id])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[VisualObjects] CHECK CONSTRAINT [FK_VisualObjects_ProductFileColumns]
GO

The trigger here seems OK - but it still needs a ProductFileId field in ProductFileData which would not otherwise be neccessary....
Forget about the VisualObjects table - it's not relevant....
TBH I'm thinking of using the original design since it's likely to be more flexible. This might create problems as the database is properly fleshed out....