US-CERT and Information Week are reporting a new vulnerability in QuickTime's handling of RTSP streams, which has been demonstrated to crash QuickTime Player on Windows and may also affect the Mac version. See the writeup by researcher Luigi Auriemma, who first announced the flaw.
Unlike the RTSP bug patched in QuickTime 7.3.1 last month, this vector works by overflowing an HTTP error buffer sent when the RTSP port 554 is closed on the malicious server, and the QuickTime client tries to switch to port 80. Sneaky.
Since we're almost certain to see iTunes 7.6 and possibly QuickTime 7.3.2 at Macworld anyway, expect another rev of QuickTime to close this hole after those versions ship -- since Apple wasn't notified in advance of this hole, it's unlikely to be caught in the pending updates, as commenter Nicholas points out (unless Apple found the vector independently).