In order to develop SharePoint 2007 workflows in Visual Studio 2008 on 64 bit, I would create a new (non-SharePoint) Workflow project. Then, I would add the appropriate onWorkflowActivated activities, etc.
I have migrated some workflow projects from VS2008 to VS2010 and they seemingly worked fine. I say seemingly because they were able to compile correctly, run correctly, and I was able to modify all the existing activities without problems.
Now, I am starting a new workflow project and I performed my normal routine, but the SharePoint activites were not on the Toolbox. I enabled all items to be shown and the SharePoint Items were greyed out (Assembly version 126.96.36.199, not the tab for 14). I unloaded the project and compared it to a project created with 2008, and I was missing the SharePoint workflow project template guid. So, I added it and could no longer load the project.
Besides working on 32 bit or developing in 2008, does anyone know any other work arounds?