Mojang is offering a special treat today - an early Snapshot! 1.4 is due to release next week, and the early Snapshot release allows Mojang more time to get the update as playable and awesome as possible before launch.
Today's Snapshot includes a helping of:
Anvil block has been tweaked (it’s cheaper to create, minor visual changes, and you can repair tools using wood/ingots/diamonds)
Bats no longer trample crops or trigger pressure plates
Redstone repeaters can be “locked” (fixed output) by having a powered repeater connected to its left or right side
Updated language files
More bugfixes & tweaks!
Grab the Snapshot today, and see 1.4 early - it doesn't include all that 1.4 has to offer, but it comes pretty darn close!