It seems like Steam for Mac doesn’t play well with Mac OS X 10.6.4. Valve itself has issued a warning to users who logged in during the weekend on Steam:
“The recent 10.6.4 update from Apple has noticeable performance issues for NVidia graphic chip owners running high performance games. If you wish to avoid this, you should consider waiting to install the 10.6.4 update until Apple has had the opportunity to address this issue. Full details of what is contained in the 10.6.4 update can be found here: http://support.apple.com/kb/HT4150. If you have already installed this update and believe your graphic performance is affected, please contact Apple support (http://www.apple.com/support/) for details on what to do.”
If you’re a Steam user and care about your game performances, perhaps you should wait for Apple to fix these problems. Should happen soon.