Apple has updated QuickTime to version 7.6.9, bringing out a whole slew of security fixes protecting against various "maliciously crafted" file formats. Apple has been working closely with security experts to hammer these out, and as a result, the software is much better protected against opening files designed to cause problems.
You can find the update in the usual places, including a Software Update client near you, or directly from the QuickTime download website. Enjoy and be safe!
Update: This fix is for Mac OS X Leopard and Windows users only, so if you're wondering why you don't see it in Software Update, that's probably why.