By default all client systems are certified for 32-bit, unless the vendor specifically requests 64-bit certification. Servers, though, are only certified in 64-bit.

This reflects the fact that the recommended (i.e. the default you get when you go to download) Ubuntu version for client systems is the 32-bit one.

This in turn obeys a variety of factors, not all of which I'm aware of. I know, for instance, that Flash support (which prior to Oneiric and multiarch support was suboptimal on 64-bit systems) meant that recommending 64-bit and expecting a great end-user experience wasn't feasible.

As Ubuntu moves towards defaulting to/recommending 64-bit, expect to see certification transitioning to 64-bit instead. A capabilities survey revealed that about 80% of systems are 64-bit capable nowadays. This becomes a necessity as more systems ship with over 3 GB RAM, which is the reasonable limit for the 32-bit kernel. Also this is made possible by the fact that the end-user experience on 64-bit has improved considerably as of late.

Two specifics about certification: Only one arch is certified because it would be quite burdensome to certify both for all systems. Also, we have seen cases where a system would work under 32-bit Ubuntu but fail under 64-bit (and vice-versa), so don't be 100% certain that it will work (you can be highly confident, but if you want to deploy on a non-certified combo (architecture/machine) please test before you buy).