I've had this happen a few times to me before, but it just happened again when I went to play Dirt 2. When you put a game in, instead of going to the standard loading 360 screen, it brings up the 360 logo and name with a white background and then stays there? If it does it then you have to go back to the Dashboard and reload the game. Here's a picture.
http://i48.tinypic.com/e9xeop.jpg
Has it happened to anyone else before?