There is a tool named TotalTerminal (formerly "Visor") for the Mac which extends Terminal.app and does exactly this. I even have control-~ mapped to the behaviour. It seems pretty configurable, and hooks nicely into the prefs window of Terminal itself. You can hit cmd-opt-F to temporarily switch the current Terminal into full-screen mode, in case you need to delve into a particular task.

Screenshot: http://imgur.com/LiYdTu2 Prefs: http://imgur.com/YCDROa3