You can fix this by restarting your Mac in safe mode. (23-Jan-10: There is an Apple help page). You do this by restarting your machine and holding down the shift key whilst it restarts (i.e. from power on). This apparently clears out the OS caches. This takes about 5-10 minutes and you'll see a grey loading bar under the apple on the white loading screen. Once you've got into OSX just restart your machine again (in normal mode) and everything should be fine.
You'll notice that the game is considerably faster now. Quite why the graphics drivers have been so bad for so long would be an interesting question for Apple. I can now run at full res on my second monitor and with medium texture settings. I tried medium settings for everything but the framerate was awful. It is a big visual improvement over everything on low previously.
21-Nov-2010: Another thing you might try, after restarting in safe mode, if you are still having issues, is to run the Starcarft 2 "Repair" application. You can find this in the directory: Applications/Starcraft II/Support. Not sure what this does entirely, but it should ensure that everything is up to date and correct.
(23-Jan-2010: Added link to Apple safe mode help. Thanks "Anonymous"!)
