Prior to 1.6 I had set the video option "limit framerate" when trying to eliminate a persistent hang problem (which finally went away when I dropped my view distance from "far" to "normal"). I also built a large (80x30) mob spawner that could go 8+ hours with nothing spawning inside. In fact, often when the mob spawner would dry up I could wander through caves for hours and never have any monsters spawn at all. With "F3 radar" I could see that nothing was spawning anywhere around me. I suspected a bug.
In 1.6 Notch un-limited anyone who had previously limited the framerate. Since it didn't matter I left it unlimited. Now my mob trap spawns mobs. And my typical framerate is only about 3x what the "limited" framerate was, so it's not linear.
I was wrong. I had one good session in the trap (4 or 5 stacks of gunpowder in a few hours) but I parked there overnight and got absolutely zero items.