Also wanted to add that you can switch back and forth easy. When Ubuntu starts up it'll stop at boot_ or something like that. At that point hit enter to keep going to Ubuntu, or type boot-game-os to go back to the PS3 OS. Going back to you're other OS is simple too, just go to system setting in the xmb (pretty sure that's it) and switch.
I was worried at first because I didn't want to format everything, but gotta say I think if you have the space every PS3 owner should do this. My PS3 currently has it's own PS3 library, PS2 BC, PS1 BC, and the ability to play anything N64 and under via emulation. It could already watch videos, connection to media servers, has an internet browser and so on. However adding Linux just adds a whole new dimension. I mean sure, these are things I can alrdy do on my comp, but what's it hurt to add another feature to your PS3? What's more is I find myself enjoying typing and accessing the web from a comfy recliner on a near 50" TV more than I was on a 20" monitor /w computer chair.