I generated a new superflat world with 1.4.3 and I went exploring around, I wanted to go back to spawn to I decided to dig down and kill myself falling into the void, when I did so, I ended up just sitting 2 blocks below the bedrock almost floating. Same thing happened in Survival and I wouldn't take fall damage just stopping on this "barrier". The only way I got through it was descending with shift in creative.