I'm installing Windows 7 inside Virtual Box running on top of Mac OS X Lion running on a Mac Book Pro (bought in December 2011 in case that matters). Should I run Windows 7 in 32 or 64 bit?
I would recommend going 32bit as I assume you are not going to dedicate more than 4GB RAM to your virtual machine, and in addition to that you won't use/need the (small) performance boost some applications gain when run in 64bit. (Note, that those are mostly scientific apps, not any daily things like browsing, email and stuff.)
When run in 32bit, you even save some memory.