The notice affects both Apple and Microsoft OS. A remote attacker does not need to do much to take over your system. In a nutshell:

A vulnerability exists in the handling of the rtsp:// URL handler. By supplying a specially crafted string (rtsp:// [random] + semicolon + [299 bytes padding + payload]), an attacker could overflow a stack-based buffer, using either HTML, Javascript or a QTL file as attack vector, leading to an exploitable remote arbitrary code execution condition.

Version 7.1.3 of Quicktime and earlier are said to be affected.

In other news the latest Mac OS X security update (2006-007) seems to have caused some interesting Thunderbird 1.5.0.9 issues, or vice versa. Nothing confirmed yet but the TB 2.x beta is ok.