Malware has been found hidden inside an innocuous 'waterfall' screensaver .deb file made available on popular artwork sharing site Gnome-Look.org. The .deb file installs a script with elevated privileges designed to perform a DDoS attack as well as keep itself updated via downloads. The dodgy screensaver in question has since been removed from gnome-look and this incident was a very basic, if potentially successful, attempt.
http://www.omgubuntu.co.uk/2009/12/malware-found-in-screensaver-for-ubuntu.html
I'll keep these brief, but another malicious file was uploaded to gnome-look, this time masquerading as a theme called 'ninja'. If you fear you've installed it head over to the ubuntuforums @ This thread to help get a fix.
http://www.omgubuntu.co.uk/2009/12/yet-more-malware-found-on-gnome-look.html
Ubuntu is popular enough to have it’s own malware!
Advice: don’t install .deb files from untrusted sources. (Gnome-Look.org is now obviously untrusted.)