One of the many sample Metro applications included with Windows Developer Preview is a Sudoku game. The command strip along the bottom is invisible unless you right-click on it. It looks as if the few native Metro apps included in the build are designed to run in the background persistently. You can't use the usual [ALT][F4] key combo to stop running applications. Switch away from them by pressing the Windows key – or by using [ALT][TAB] combo.