While Linux was the first operating system to have strong x86_64/AMD64 support, there's been Ubuntu 64-bit images from the start, and most hardware for several years has supported 64-bit software, Canonical / Ubuntu have always recommended the 32-bit version of Ubuntu over 64-bit (in terms of when going to the download area of Ubuntu.com, etc). With Ubuntu 12.04 next April, this will finally change so that Ubuntu x86_64 is the recommended version, but there will still be 32-bit images offered.
The reason for finally going 64-bit by default is that multiarch will provide superior 32-bit compatibility for packages, Adobe now offers a mainline 64-bit version of its Flash Player, and that UEFI support is only offered in the 64-bit version of Ubuntu.
Most people that tended to stick to 32-bit Ubuntu even on 64-bit capable hardware was due to 32-bit binary software issues (now largely gone thanks to multiarch improvements and 64-bit Adobe Flash support along with 64-bit OpenJDK) or just using 32-bit since it was "recommended" by Canonical. This is while Ubuntu 64-bit is much faster than Ubuntu 32-bit.