Originally Posted by cyccommute

If you had a clear tube without threads at critical points, i.e. a really long fork, you might not have problems. But if there were threads under where the stem clamps to the fork, I'd be much more cautious. Since the threads are cut onto the steer tube, material has been removed making the area thinner. Also if there is a keyed washer slot cut in the tube, the fork would be weaker still. It might work or it might fail. Failure of the steering parts of your bike isn't good. I'd think about it long and hard before doing it. You can get lots of threadless forks for cheap (check for old ones at your local bike shop).