I had this happen to me under the following condition:
With a web site (as opposed to a project) in VS2008:
1) I copied an existing / functioning usercontrol (as in CTRL + Click, Drag, Drop)
2) I modified the copy slightly (attached a listview's datasource to a different stored procedure)
I fixed it as follows:
1) Create a NEW user-control (add new item in VS)
2) Open the .ascx I had originally copied, and this time just copy all of the mark-up, and paste it into the new user-control's .ascx.
3) Same for the .ascx.vb file -- open the one to copy from, copy all of the code, and paste it into the new user-control's .ascs.vb file
If you just look at the old and new files, there's no explaining it -- they appear identical. The difference for me was, copying the file failed, whereas copying the contents of the file succeeded.
I suspect this may have something to do with partial classes, and the fact that you can't access all of the parts when working on a web site. I think when it comes to projects, you can access all of the parts -- and maybe if that were the case, I'd be able to see what was causing the problem.