>Could the other process be creating a "filtered result set" in a Select-SQL statement?
>I've found that a table in such a state (acting as a cursor but really a filtered
>result set) cannot be closed.

I've found it to be the other way around, a filter cursor is closed with close tables, a normal cursor isn't:

Clear
Close Tables All
Close Databases All

Set Safety Off
Create Database d:\test\temp.dbc
Create Table d:\test\test.dbf (iID I)
Use in Select("test")

* will generate a filter cursor
Select * from test into cursor curTest
Set
? "RESUME to Continue"
Suspend
? Dbf("curTest") 
Close Tables
Set
? "RESUME to Continue"
Suspend

* will generate a "normal" cursor, no filter:
Select * from test into cursor curTest nofilter
Set
? "RESUME to Continue"
Suspend
? Dbf("curTest") 
Close Tables
Set
? "RESUME to Continue"
Suspend
Set Safety On