This blog is very late to be publishing at this point (given I ran into this a long time ago) as many know how to debug a MOSS workflow. All you need to do is:
- Make sure that you get a debug build in the gac.
- Restart IIS or recycle the SharePoint app pool, which ever you prefer.
- In Visual Studio >> Debug >> Attach to Processes… >> select the w3wp.exe.
Now, do not forget to Attach to only the Workflow Code Type.
Apparently you cannot debug workflow and managed code at the same time. Otherwise, you will find yourself closing Visual Studio, bouncing IIS and re-opening Visual Studio just to debug your workflow… Doh!