Apple sent out a bunch of updates today, not all of them related to their product announcements at Macworld. They released a security update for Quicktime. Quicktime 7.4 patches 4 security vulnerabilities.
Quicktime required a reboot on my Macs, as do all Quicktime updates. On Windows I have the iTunes+Quicktime combo installed and that also required a reboot.
The update is available through Apple software update or as individual downloads from Apple’s download page. There are versions for Leopard, Tiger, Panther and Windows.