yes, in certain scenarios (heavy rider+badly designed and slippery dropouts+poor quality skewer) the wheel can pop out under heavy braking. but if everything is designed/set up properly i can't see how is this possible.
It's possible because some bike designers very clearly don't understand the physics of disc front brakes. Why else would they specify a maximum rotor size? (Specifying a minimum would make much more sense.)
There's a marketing-led rush for road utility and touring bikes to be fitted with discs at the moment. Road bikes will follow when Sram unveil their racy lightweight discs sometime soon. I think it's possible we'll hear more horror stories of ejected wheels and folding fork blades before the issue is well understood and resolved.
Having said that I'm very happily running Hope Mono M2 twin pot discs on my mountain bike. It has very meaty Fox Talas forks though, and I have 180mm discs rather than the standard 160mm.