The community documentation has this recommendation on its 32bit_and_64bit page (last edited 2010-07-29)
"What should I choose - 32 or 64 bit?
Unless you have specific reasons to choose 32-bit, we recommend 64-bit. "
A consistent viewpoint would certainly be better.
IMO it would be useful if the Ubuntu site could make clear that this decision on Ubuntu is a little different from this decision on Windows. On Win7 64 bit works just fine, and almost half the Win7 installs globally to date have been 64 bit (according to Microsoft); conversely the 32 bit version really wont work with memory above 4G. On Ubuntu, there are still some issues with 64 bit (getting 3rd party apps to work IMO) and PAE means that the 32 bit can (in most cases) address the memory above 4G in your system just fine.
I have 64 bit, but I somewhat regret that decision, and if I was choosing again, I suspect I would choose 32 bit.