As the ladies have said, it's very possible. I do it all the time getting started through intersections, especially with my new Keo Grip cleats that require an annoying level of accuracy to engage (with the older style, so long as the pedal was right side up, I was ok). Just don't put all of your weight on the unclipped side, because you won't be stable. Focus on doing most of the pedaling with the other leg.

When I was new to clipless I did the unclip and rest on top approach to a lot of traffic lights. If it turns green, stomp back down and clip back in. If not, then you don't have to worry about unclipping. Something to think about that may be a little bit more common with super easy to engage pedal systems like Speedplays -- If you do this, be sure you don't inadvertently clip back in. That happens sometimes and then you don't double check that you're still disengaged before you stop and topple over.