The articles from Microsoft are available here and here.  It sounds like some good fun….

FTA:

This security update resolves a privately reported vulnerability in Microsoft Virtual PC and Microsoft Virtual Server. An attacker who successfully exploited this vulnerability could execute arbitrary code and take complete control of an affected guest operating system. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights.

I know I’ll be updating all my machines…!