Unity is yet another theme proposal for inclusion in Ubuntu 10.04 Lucid Lynx.
Where Unity differs from 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. (*I think they mean ‘Dust’?)
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