The 1.4 "Pretty Scary" update for Minecraft on the PC was pushed back a day after Mojang discovered and fixed a serious bug involving suffocating animals. A second pre-release version of the update is now available to download though.

Here are the fixes that are included with 1.4.1.
Fixed mobs glitching through wooden floors
Fixed players glitching through Beacons
Fixed Redstone Repeaters getting stuck in o ono state
Bats spawn less frequently
A number of minor font fixes
Activated the Wither painting
Fixed wet wolves looking way too scary
Tweaked naming costs in the Anvil
Tweaked enchantment points reward for smelting
Fixed bug that allowed any item to be placed in the armor slots
Fixed mob spawners spawning too quickly