MySpace Shockwave Flash Hack
These threats are interesting because they are not “classic” threats where you must download content, save it, and execute it from your machine. These threats do not modify your local machine, but they replicate solely within the context of the service provider (in this case MySpace). So, they don't infect your machine per se, but they do infect your virtual space on the service provider's servers; leaving nothing for local machine security products such as antivirus or desktop firewalls to scan for after the space has been infected.
Thanks to http://www.chaseandsam.com/ and kinematic.theory for the initial heads-up.