While I completely support your right to get a refund for this bug (as several have noted, it's really frustrating and renders the game unplayable when it's active) I'll note that after a couple of days that problem magically cleared up for me. I have no faith at all that it won't come back as magically as it went away, but if you'd rather have the game than the money you might want to wait a couple of days and try again.
Another possibly useful debugging point: I was trying to play a bit holding down the "w" key and never letting go (i.e. always keeping moving) and I realized that the spacebar (jump) stopped working while I was still moving. This suggests that the point of breakage is the keydown event of the last w that works, and that keyup events are handled fine.
Tails: Thanks for the pointer! Having now installed lwjgl-2.8.4 I can add the datapoint to the pool that it didn't help :} :{.
Can anyone point me at how to update LWJGL? Based on the comment thread that seems like a chicken sacrifice worth trying.
I'm getting this too. 10.8.2. It's happened on all version of Minecraft that I've played, but is now happening almost everything time I use the "w" key (hold down "w", get to where I want to go, stop holding "w", "w" now doesn't do anything). On interesting thing I didn't see anyone else mention: the program does respond to the SHIFT key, crouching in the way I'd expect from stealth mode. But it doesn't respond to any other keys. The game is currently unplayable; Cmd-Tab twice every time I move really detracts from the immersive experience :} :{.
A reboot didn't help. I haven't tried re-installing Minecraft (don't want to lose my game, and not sure how to save it).
This is extremely annoying when debugging. When will it be fixed?