QuickTime 7.6 is in the wild. Released today via Software Update and on Apple's support site, the new version "includes changes that increase reliability, improve compatibility and enhance security." It also makes your whites whiter and your brights brighter while helping you avoid embarrassing social situations.
More details via the update page:
Video: Improves single-pass H.264 encoding quality; Increases the playback reliability of Motion JPEG media
Audio: Improves AAC encoding fidelity; Audio tracks from MPEG video files now export consistently
Application Support: Improves compatibility with iChat and Photo Booth
As far as the security updates are concerned, the word from Ryan Naraine at ZDnet is that this patch closes several potentially serious holes that could have been exploited by malicious URLs or custom-payload movie files to execute code on target machines. Four of the seven issues addressed in this update were reported to Apple via the Zero Day Initiative, which encourages responsible disclosure of vulnerabilities by providing cash bounties for the researchers involved.
Thanks to everyone who sent this in.