(In reply to comment #13)
> Is this still an issue with current Mesa git?

I've just tested WoW on my M66GL (RV535?) with the latest Mesa which has head:

commit 54ff536823bf5a431efe1f2becdb21174c146948
Author: Marek Olšák <maraeo@gmail.com>
Date:   Sun Dec 2 22:22:39 2012 +0100

    r300g: increment num_z_clears only if we have Hyper-Z

And it seems to work OK; no artifacts seen. Unfortunately, the new WoW login
screen didn't have the problem that the previous one had anyway.