On the download page, it will ask about the 64 bit version.

http://www.ubuntu.com/getubuntu/download

The only OS thus far that I have seen properly blend 32 and 64 bit fully for install (and 2 different CPU architectures) is the upcoming Leopard version of OS X. Being that they ship the OS out on a physical disc means they can do this easier, without worrying about wasting bandwidth on a "fat binary" installer.