And The Reason Why The Metacity Window Buttons Are On The Left In Ubuntu 10.04 Is...

...from Mark Shuttleworth himself:

The default position of the window controls will remain the left, throughout beta1. We're interested in data which could influence the ultimate decision. There are good reasons both for the change, and against them, and ultimately the position will be decided based on what we want to achieve over time.

Moving everything to the left [...]