I've run into this problem about 4 different times today, each time with a different block. First, a door, then a fence gate, then a trapdoor, and currently a portal. each time, it's the same - I get stuck, looking in one direction, unable to move at all or look around. Breaking blocks does not help unless another player in SMP does it. My SSP test world is currently inaccessible due to this bug, and I've had to ask friends to break me out of such situations multiple times.