There are three methods of fullscreen handling on the device right now:

The browser has a toggle button in the lower right, which removes all toolbars. When the screen is touched, a transparent button is placed on top of the lower right corner, which can be used to exit fullscreen. This presumably involves some wizardry and is probably not recommended as a standard way.

Other applications have a toggle button on the toolbar in the lower right, which only removes the titlebars. The toolbar always stays on screen, so you can toggle back to normal mode.

Games and similar applications always run in fullscreen and don't have a way to exit fullscreen mode. However, they should provide a close button in the upper right corner, so that the user knows how to get out of it again. Bonus points for a way to access the task switcher in the upper left corner. Bounce does both very well, whenever you touch the screen it pauses the game and displays the controls. Camera and Media Player only have back buttons in the upper right corner in fullscreen modes, which gets you back to the non-fullscreen part of the application.