To the best of my awareness, there shouldn't be an issue with backups - only exception might be if you use a data type or some other entity that's specific to a newer version of SQL, and then try to restore to an older version. But aside from that, I have no knowledge of any issues on backup/restore between 32 and 64 bit for the same general version of the product.

Now...there's one other issue with 64 bit. Again, if you have plenty of memory, then 64 bit is the way to go. However, (and not to complicate things), there might be times where you have to disable 64-bit support and use 32-bit for very specific tasks. For instance, a few features in SSIS only work in 32 bit. The total # of instances where this occurs isn't high, but there can be times where you might need a 32-bit driver for something or might need to set a few extra options.