Unity is yet another theme proposal for inclusion in Ubuntu 10.04 Lucid Lynx.
Image link since broken
Where Unity differs from other proposed GTK themes is that it aims to offer a complete “fluid” look covering GTK, Metacity, Lock screen, splash screen, etc. The GTK theme itself is modelled on the ‘Dusk*’ theme.
In keeping with current trends you may not be shocked to hear that the theme is but mock-up only currently.
You can read more about ‘Unity’ @ http://ubuntuforums.org/showthread.php?t=1397770
(*I think they mean ‘Dust’?)