I had a Visual Studio 2008 web part solution that I upgraded to a Visual Studio 2010 solution. It looked like everything was going to be ok, but I got the following error in the ULS when trying to add the web parts to a page:
09/09/2010 14:08:36.82 w3wp.exe (0x269C) 0x1998 SharePoint Foundation Web Parts tkqj High WebPartAdder_InvalidDwpUrl
Checking the solution again the web parts where added as modules and not web parts to the solution.
I ended up deleting the web parts (except the source code of course), adding new web part items, and adding the deleted code. Now it is working as it should