Have you found that some keyboard shortcuts in Photoshop CS5 (Mac) haven’t been working? Or are some tools acting strangely, as if the Option or Shift keys were pressed? It turns out that Mac OS X 10.6.5 introduced a bug where modifier keys might stick; for example, you might always see the Hand tool, as if the spacebar was pressed, and you might be unable to switch away from the Hand tool. One way I experienced it was when I tried to press the Command and Spacebar keys to get the temporary Zoom tool, but nothing would happen.
The bug was also in Mac OS X 10.6.6, but Adobe reports that Apple finally fixed the problem in the Mac OS X 10.6.7 update. So while there have been various workarounds that involve tracking down software conflicts, now that Mac OS X 10.6.7 is out, the fastest and simplest solution is to download and install the Apple update. As usual, you can run Software Update to get the new version, or you can download the Combo installer from the link above.
While the problem isn’t mentioned in Apple’s release notes for 10.6.7, this issue is related to a change that Apple made to an API in Mac OS X 10.6.5, as explained in this forum post. A number of Mac applications were affected, but Photoshop was the most prominent one, and there are several threads discussing it on the Adobe User-to-User Forum for Photoshop.
Update: OpenType issues introduced with Mac OS X 10.6.7
Upgrading to 10.6.7 is no longer a no-brainer. Apple has apparently introduced a new bug related to OpenType fonts in PDF files generated by Mac OS X 10.6.7, so if you use OpenType fonts (and who wouldn’t, since they’re modern and cross-platform) you may have problems printing or rendering from them in 10.6.7. I suppose it comes down to whether it’s more important to you to have your keyboard shortcuts working in programs like Photoshop (in which case you’d upgrade to 10.6.7), or whether you need reliable handling of OpenType fonts (in which case you’d stick with 10.6.6).
(Update to this update: Apple has fixed this OpenType issue in the Snow Leopard Font Update, so make sure you run that update after updating to 10.6.7.)