This afternoon, Apple released the latest version of QuickTime — 7.6 — which includes updates for encoding quality, reliability, compatibility, and security enhancements.
Media improvements include better single-pass H.264 encoding quality, better playback of Motion JPEG content, improved AAC encoding fidelity, and improved audio export from MPEG video files.
Click here for details on video improvements: http://support.apple.com/kb/HT3292
The release notes for the new update also include a variety of security fixes. Click here for information on security improvements:
Windows download: http://support.apple.com/downloads/QuickTime_7_6_for_Windows
OS X 10.4 download: http://support.apple.com/downloads/QuickTime_7_6_for_Tiger
OS X 10.5 download: http://support.apple.com/downloads/QuickTime_7_6_for_Leopard
Final Cut editors should wait at least a week before upgrading to make sure this version does not introduce new problems. Also, never upgrade QuickTime in the middle of completing a project.
Here’s an article I’ve written that explains my thoughts on when to upgrade.