I'm not going to get into the details on why Mark Shuttleworth insisted on moving the window controls in Ubuntu from the right (which they've been at since October 2004 and in general on Windows and Linux since 1994) to the left, but as Ubuntu 10.04 Lucid Lynx was released, the window controls are staying on left.

To adjust the controls from the left back to the right, you can change the theme away from 'Ambiance' or 'Radiance' or run the following command in the Terminal (Applications > Accessories > Terminal)
gconftool-2 --set /apps/metacity/general/button_layout --type string "menu:minimize,maximize,close"