I've been trying to play some linux supported games on ubuntu 12.04 using steam (mostly Dota 2) and I've been having the same problem every time with every game. As soon as I alt-tab out of fullscreen mode the game just vanishes. Its still running in the background, it still shows up on the system monitor but I have no way of accessing the game again. It doesn't show on the task bar, doesn't show on Alt-Tabs... just weird.
Granted, I am a complete linux noob (as in I google absolutely everything I do) so I might be missing something incredibly obvious here... o.O