I believe the architecture type (x86 vs x64) is abstracted away for you when making .Net programs, but are there any other considerations that can cause problems?
MSDN had put up a little paper regarding issues of porting 32-bit applications over to 64-bit execution environment.
http://msdn.microsoft.com/en-us/library/ms973190.aspx
Two other bloggers had previously wrote about 64-bit development when they were working in the CLR team