I ran across this with RC1 also (x64 GNOME). This bug does not affect the GNOME 3 Preview/GNOME-Shell (while available, it is not installed by default).
Improvements with RC1 over M6 (noticed so far):
DRI/DRI2 is available for AMD HDXxxx (up to HD5xxx) GPUs using the r600c FOSS driver by default. (While this was possible with M5, it required both the X.org from Factory and a post-M5 kernel (either from Factory or a custom kernel); this is no longer the case.) For non-gamers, this is very much a welcome surprise, especially if you are looking at one of the heavier desktop environments (GNOME or KDE in particular).
GNOME-Shell is up to 2.91, and is present in the standard repos (the rest of the GNOME 3 Preview should soon follow suit). If you have DRI/DRI2 (and unless you have a graphics chipset among the outliers, you should) and are looking toward the future of GNOME, check it out.
As was posted during M5, KDE 4.6 is out (the entirety of KDE SC 4.6.0 is in the M6/RC1 repositories). Like GNOME, DRI/DRI2 support is pretty much a necessary.