I actually did this once, just for fun- on a single front fork. Disc fork, disc wheel, a spare pair of v-brakes and mounts on the fork... hmm....

It did actually work, but I nearly went over the bars on a downhill. When checking the front wheel afterwards, the spokes looked like they were stretched (ie showed signs of stress). I wouldn't reccomend it.

Also, like coldfeet said, properly set up discs or v-brakes can lock the wheel. Even the cheapo Promax mech. disc on my Impasse's rear wheel can lock it up in a pinch.