A common problem with 64bit development with Revit is the inability to use the Microsoft Jet OleDB driver to read and write to Excel files easily.
In the Getting Started With Revit API.doc file in the 2010 Revit SDK it states:
Note that there are some Microsoft components (such as OleDB provider for Microsoft Jet) that are not supported for 64-bit. All SDK samples are set up to work with either 32-bit or 64-bit Revit, except those samples that have dependencies to unsupported Microsoft components.
Basically, the problem has been that the 3rd party DLL file which is supplied by Microsoft did not have a 64bit version.
While this problem has not yet affected me in my development, I know a few people have mentioned it so I thought I’d pass on something I read today: How to get a x64 version of Jet? on MSDN blogs.
Apparently the beta of Office 2010 has the 64bit dll file everyone has been looking for, it is available HERE.
I hope this helps you out if you have run into this problem.