Here's my code for a very very (very) basic accelerometer based application that I was working on a couple of weeks ago. It's my first Cocoa app ever, as well as my first jump into any kind of game programming. It's just a ball that accelerates when you tilt around your iPhone, and hopefully collides with the walls that are around it. Because of my inexperience with basic collision detection, sometimes the ball breaks through the barriers, but you can get the basic idea of what I was doing from this.
As you can tell from the source, getting to the iPhone's accelerometer data is really easy, just overriding a method that is provided by Apple's UIApplication framework, which every iPhone application has access to. Although it's not documented on the internet very well at all, the good folks at iphone-dev/iphone-uikit channel provided me with help with getting to the accelerometer data (thanks kroo).
I might post a video if people still don't believe that the accelerometer is really this sensitive for those who haven't started with 3rd party apps yet.
I'd like to mention that the developer in the Youtube video definitely has some awesome programs, and I only wish I had the cocoa expertise to write them.