I had the problem of not able to debug Sharepoint in Visual Studio 2008 awhile ago, and glad that Aaron shown me how to do it by doing some tricks. What happened was the PDB file might not have been deployed to the assembly GAC.
What you do is:
- Start, Run, %systemroot%\assembly\gac, [ENTER].
- From there, go up one folder, then into GAC_MSIL folder.
- Paste the pdb file with the DLL, you will find yours amongst the folder names.