In my humble opinion... anything that gets kids out from in front of their GameBoys and outside doing physical activity is awesome! Pretty much any activity associated with childhood carries it's risk of injury. Riding bikes (my daughter took a huge chunk out of the back of her thigh last year doing this), climbing trees, swimming (had a friend die at our senior picnic no less), roller blading (my sister broke her nose once, it's still a bit crooked) etc etc etc. You can drive yourself up a wall worrying about the risks, or you can encourage your kids into activities, all the while firmly stressing safety. (helmets, knee pads... whatever it takes for the activity they're taking part in)
As far as the trampoline is concerned specifically. I'd be more concerned that it doesn't invalidate his homeowners insurance. Personally, it's specifically mentioned in our insurance that we can't have a trampoline. If someone gets hurt and sues us, we are fully liable and they'll cancel our policy.
I for my part have a problem with the Heelys that all the kids are wearing. Not in and of themselves, but the fact that they've turned every store and mall into a skating rink. Not cool.