Like many other unfortunate users, I went ahead and installed Google Hangouts android app right away when it became available on Google Play. Unfortunately, I did not realize it overwrites Google Talk completely, with no way to downgrade. And it misses a lot of features, like being able to see your online contacts list (pretty basic IM feature eh?).
Fortunately, there is a way to revert the upgrade. If your device shipped with Google Talk, you can simply uninstall Google Hangouts. That will replace the "factory version" of Google Talk back to your phone. You can always reinstall Google Hangouts later :)
On Samsung Galaxy S II, applications are uninstalled from Settings -> Application Manager -> app -> Uninstall.