In case you installed the 10.6.8 5770/5870 patch for Lion (which enables DisplayPort but breaks Quartz Extreme, no longer recommended!), here is a patch that restores the ATI graphics drivers to their original Lion state. It’s essentially a patch uninstaller.
Note: Only install this package if you are running the Lion original release (OS X 10.7).
You can find the revert package (patch uninstaller) here.
For the most up to date info about Lion and 5770/5870 cards, see this post.