I know of the on_minimized/on_maximize callbacks, however, is there a callback that is triggered when the window looses focus so I can automatically pause the game?

not shure, but try to look into;

http://www.conitec.net/beta/on_message.htm

on_message functions,
there you should be able to grab the unfocus event

Thanks I believe its good now.

The 'window_focus' variable would probably be neater.

Yes but, its just a variable so I'd have to check it every frame... (which I might end up doing if my post here isn't resolved).

So I could use the WM_SETFOCUS and WM_KILLFOCUS messages to check for that...