I've used linux on the ps3 for a bit now so I'll give you a few thoughts to think about. On a side note, I've been using some form of linux/unix for years now, so it is certainly possible that things that seemed straight forward to me might not be as clear to a newer user. First, linux users actually have access to multiple SPE/SPU cores (six) as well as two hardware threads for the PPU. As far as I'm aware, the linux kernel itself doesn't utilize the SPE cores, but there are ongoing projects to develop drivers (ex. graphics driver) that would make use of the extra hardware.
Second, ubuntu certainly works (and so does Red Hat's Fedora core, Yellow Dog, Gentoo, etc). As I mentioned above, the SPE is not the primary processor in linux and for the most part the hardware was supported long before the ps3 was released. You'll actually find that a large portion of the software (including firefox) works perfectly on the ps3. The problem that you'll run into, however, is sluggish performance, which leads me to my next comment.
If you don't plan on developing for the ps3 (I would recommend Fedora Core or Yellow Dog if you do), then I would strongly suggest using Gentoo. After a fresh install, that is the only distribution that provided me with a smooth/lag free user experience. The main reason for that is gentoo doesn't install a lot of extra stuff that you don't need or will ever use.
If you do go down the ubuntu/fedora core route, then you'll want to disable a large portion of the services/programs that get automatically started. If you don't, the small amount of ram in the ps3 will just kill your user experience. Luckily, there are plenty of guides on the internet describing what can safely be disable and how to do it.
In general, once the OS is setup (and unneeded programs are disabled), the ps3 serves as an ok linux box. It will handle basic things (web browsing, email, etc) just fine. That being said, the true redeeming quality is being able to develop for and use the SPE.
If you're at all interested in that MIT has a course on multi-core programming and the labs are excellent tutorials for ps3 programming. Anyways, gotta run now so please forgive any incomplete thoughts/grammatical errors in the above post. I was in a hurry when I typed it.
