Apparently due to some usability tests, the newly available Ubuntu Lucid Lynx Beta has the window buttons on the left side. This is fine for many and if you are used to keyboard shortcuts you wouldn't care about placement of buttons anyway, but in case you want to have the old style back, this is for you.

Open a Terminal and run the following:

gconf-editor

This opens a registry-like interface. Navigate to /apps/metacity/general and find the key named "button_layout". The key documentation tells us

The value should be a string, such as "menu:minimize,maximize,spacer,close"; the colon separates the left corner of the window from the right corner, and the button names are comma-separated.

Just replace the value with the suggested string (spacer can be omitted), and the buttons are back at the right side. Keep in mind that in the new default Ubuntu theme the minimize and maximize buttons are switched too.