Pretty cool demo. I use Vimperator[1] to do most of my browsing. The experience is quite good. It would be interesting if Tab Candy will have apis to drive these features. Then vimperator commands could call them like,

:group Work - current tab into the Work group

:switch Todo - switch to the Todo group

etc

The features look great. But after using Vimperator, I prefer keyboard alternatives to the drag-and-drop stuff shown.

[1] - http://vimperator.org