Depends which driver you are talking about.
On radeon the 3d support is fully integrated and co-exists with 2d / video acceleration. On radeonhd the same acceleration code is available in the quick & dirty 2d accel branch which should get merged to master pretty soon (maybe today).
I haven't looked at the wiki today but I wouldn't be surprised if 3d is still flagged as "experimental" because of the 2d/3d co-existence issue, but that will go away as soon as the q&d accel branch is merged.
I think you're talking about radeonhd from the quote, so if you download and build again as soon as the q&d accel branch gets merged the restrictions should go away.
Note that you also need pretty current mesa (7.1RC3 or higher) and drm code to get full 3d accel on a 5xx, and unrelated changes in mesa also require a new X server in order to keep AIGLX working.
The good news is that the combination should run pretty nicely, and EXA 2D performance seems to be real good in conjuction with the glyph cache improvements in a 1.5-ish X server.