I thought the PS3 only allowed the user to run Linux in a mode such that the user only had access to one processor and zero access to the graphics processor (well, no acceleration, but you do have access to the display buffer). Maybe this was hacked? I never heard about the PS3 being hacked, but I really don’t pay attention to those things as much as I used to.
Also, this article claims that the Cell processor is a GPU ... it is not a graphics processor ... it is multiprocessor system-on-a-chip. You could use the Cell to render graphics, but that would be a waste seeing that a perfectly fine GPU exists in the PS3.
I think Sony pitched Linux support since some users were installing emulators and running old games for free. I know there were a few Linux distros available that made this a piece of cake (never tried those). Sony is now posting a lot of old PS1 titles for sale on their store.
Sorry .... after a quick check, you do have access to 6 of the 7 processors in the Cell when running Linux.
The PS3 runs Linux under a hypervisor, and like any game it only has access to six of the seven SPEs plus the main PowerPC unit. Yes, RSX access is still restricted.