Unfortunately, unless someone finds a hole in Sony's hypervisor, it won't ever be possible to use hardware video acceleration in Linux on the PS3 without hardware modification.
I'm not sure exactly what the motivation is...my guess is that Sony made it possible to put Linux on the PS3 because they would dearly love to hurt Microsoft by getting someone to switch to Linux. And, of course, getting a little community cred doesn't hurt.
I won't bash the fact that there is a hypervisor, since I think it does something valuable: it makes it possible to install any PPC version of Linux, without special modifications. Without the Hypervisor, Linux would have access to the entire hard-drive, and it would be possible to overwrite the GameOS when installing Linux, so Linux distributions would have to be carefully tailored to the PS3 before new Linux users could install them...one would have to take care to avoid formatting the drive when installing Linux. With the hypervisor, Linux can't see the GameOS partition, and can't write to it, making Linux installations safer.
The downside of the hypervisor is that Sony decided to make it impossible to use the video card from within Linux, which means the PS3 Linux experience is disappointing, unless you wanted to use your PS3 for word processing. It makes a decent low-powered server, but not a very good home theater PC. My guess would be that they were afraid (perhaps justifiably, though I don't know enough about the subject to say) of the potential that someone might write a PS3 on PS3 emulator (or a WINE-like API implementation), which could result in piracy before the system becomes obsolete.