This has now changed with Snow Leopard, but even then it's only one video card:
""While Apple has previously included graphics cards that have contained hardware support for H.264 decoding, the company has only recently taken advantage of this hardware acceleration. Mac OS X Snow Leopard's specs officially acknowledge this support but it appears to be limted to the NVIDIA 9400M graphics processors found in recent Apple laptops and desktops," Kim reports.
"Unfortunately, it does not appear that this support will extend to older video cards," Kim reports. "Hardware decoding of H.264 video improves the performance of video playback while leaving your computer's CPU free for other tasks."
Also, it's not like their hardware can't support it right now; Ogg video in Firefox 3.5 runs just fine on my Mac.
And right now that's going a long way toward killing Theora; mobile devices need a power-efficient hardware decoder to do video, and if they don't get one for Theora they're not going to run everybody's battery life into the ground by using software decoding.
It means, that if TI/Qualcomm/etc start developing Theora DSPs this year, they will be on the market and in the devices before HTML5 goes mainstream.
Standardizing on Theora is pretty good incentive to have it hardware accelerated.
Interestingly, when YouTube supported only Sorenson, Apple didn't care for that and went on ignoring YouTube. Why it is suddenly a problem?