Could you hotwire the Magic Trackpad into the Air's power lines? Somehow... :}
probably. The trackpad (and the keyboard) survive quite nicely off the 3V it gets from 2 AA batteries. Since I'm not running a screen the 45W power supply could easily power both in addition to powering the system. I'd just need to bleed off some from the power rails and convert the 14.5v at the magsafe to 3v for the trackpad/keyboard. The most annoying issue at the moment is that since Apple started using non user replaceable batteries their laptop motherboards no longer have PRAM backup batteries....since theoretically there is always the system battery connected. It's only annoying in that when you boot up for the first time the system clock resets itself. If you're networked it quickly syncs with the Apple time servers though so it's no deal breaker (unless you're Steve Jobs perhaps). As long as the power is connected to the magsafe the PRAM stays.