http://bugs.winehq.org/show_bug.cgi?id=24274
--- Comment #4 from Callum Fare userof@hotmail.co.uk 2010-09-05 08:58:08 CDT --- (In reply to comment #3)
This is almost certainly a driver bug, please file a bug at bugs.freedesktop.org instead. Note that radeon_texture.c is part of the "classic" r300 driver, while most active development will go to r300g. It may be worth giving r300g a try.
Ah, thank you very much, using the r300g driver to run Steam means that it now works perfectly! I'll close the bug and write up a HOWTO for other Radeon users.