Apps can take over the screen, like a game, but I dunno about simply hiding the menu bar.
I'd like to be able to put the menu bar inside the active window. That helps a lot when you've got multiple screens.
Ironic that Windows does it that way and has crummy (to put it mildly) support for multiple monitors, while Macs have almost always had multiple-monitor support by anchor the menu bar to just one location.
-Rob