Ditto Unworthy and TheSlav.... Hot hair dryer works wonders. This is accomplished by holding the hairdryer (on hottest, high setting) directly on the decal, about an inch away from the surface, for about 15 seconds. It then peels off easily.

This also worked to take off my "TOYOTA" decals on the gate of my truck, and just about every decal I've ever tried it on.