>I've managed to populate the items collection using a shared datasource as a staticresource, and using the combobox extensions to make a comboboxdatasource. I've got the selectedvalue binding properly so I can save choices I make with the combobox.

In WPF, that's usually the hard part.

>Problem now is I can't set the initial value properly. In the datagrid, my textblock in the columntemplate shows the correct value from the datagrid's datasource, but the combobox in the edittemplate always starts off blank. If I click on it I have my items populated and I can select one and save the data, but the next time I go to edit that cell the combobox will be blank again.

I think there is something different about Silverlight on this one. But I'm not sure.

Can you show me your XAML for the Combobox column, I may be able to figure it out.