And it will pick up every shred of magnetic crap around. Not saying it's a bad idea, but I've worked with magnetic tips on CMM systems, and they pick up small metal shavings and other debris and it throws everything off... A screw mount would be preferable, if the threads are course enough to resist cross threading, maybe...
I'm sure Apple will find a way to 'innovate' themselves out of that problem. Heck, they could have almost threaded the lens housing on the latest Touch and already have lenses for that due to the distance the lens already sticks out.
Point: Whatever the choose, it has to be durable, and easy to connect properly...