Apple's first security update of 2007 is recommended for all users and improves the security of QuickTime. The update fixes the buffer overflow issue in QuickTime's RTSP URL handling.
From the Apple security Web site:
Security Update 2007-001
Impact: Visiting malicious websites may lead to arbitrary code execution
Description: A buffer overflow exists in QuickTime's handling of RTSP URLs. By enticing a user to access a maliciously-crafted RTSP URL, an attacker can trigger the buffer overflow, which may lead to arbitrary code execution. A QTL file that triggers this issue has been published on the Month of Apple Bugs web site (MOAB-01-01-2007). This update addresses the issue by performing additional validation of RTSP URLs.
As always, I recommend waiting a minimum of 72 hours before installing any software update.