Iphone Screen

The nice thing about electronics is that as they shrink they seem to become more durable because moving parts are eliminated and disparate parts become integrated. The iPhone can take a lot more abuse than the first-generation iPods do, mostly because of the flash memory. Ars Technica published an article a while back on what it takes to destroy an iPod Nano. They're sturdy little buggers.

Yes, the screen is the most likely component to break. Obviously I haven't done any durability testing, but you ought to be okay if you buy a case and keep it in a pocket close to your body with the screen facing inwards. Then again, I'm not sure I'd want to carry electronics while doing BMX, let alone listening to music. Maybe just leave it in the car.

You could always buy an iphone case that has screen protection. I bought the SeeJacket Crystal which allows you protect the screen while still maintaining it's functionality. The site is german, but I bought it on amazon for like $30.

Get the sturdiest case you can, but there is no guarantee that if you land with your full weight on the phone that it won't damage it.