Hehe, I found out about the game N+ thats been released on the Xbox Live arcade this week, and found that theres the free version to play on the net, called N.
Its pretty addictive actually.
The concept is to collect the gold coin type objects to increase your time on the level, and to get to the end which simply involves a door. To open the door, you have to first flick a number of switches throughout the level.
May sound easy, but the game is purely physics based, so you can jump up walls like a ninja, and expect drops to kill.
You should give it a try