I happen to have an RV200, which I tried with F15BetaRC1, and while it was generally unusable for many reasons, the fonts were not corrupted as they are with RV250/280. So, I think that may support the idea of there just being a bug in the R200 implementation.
Yes, this was at time when gnome-shell run with nomodeset (e.g. without KMS). It was slow, but it was displayed properly.
Adam Pribyl
I just booted the LiveCD as is on the RV200. KMS was enabled and the speed wasn't a problem. It had other severe rendering/shading issues, plus something I did completely hung the box.
Getting back to RV250/280, I saw that Red Hat BZ 679579 has a reference to freedesktop BZ 34280, which sure looks like the same problem (at least in the tuxracer example). More importantly, one of the guys there has a patch against mesa for it which seems to work for him. Perhaps that could be investigated to find out if it applies to Gnome as well?