How do I get the American Flag back on my menu bar?

in the Language and Text, under Input source, you need to select one or more input sources; you also need to select Keyboard and Character Viewer on the top of the input source list.
As far as I know, the flag is displayed only when you select more than one input source; differently, the icon you see is a generic one.

This is the icon you see when just an input source is selected.

Generic icon

This is the icon you see when you selected two or more input sources; in this case, the selected input source is "Italian Pro."

"Italian PRO" icon