The latest install-at-your-own-risk Minecraft update (snapshot "12w34a") is now available over at the official Mojang blog. Once installed, several fundamental changes are made to the way maps and nether portals function. You'll also get some new items, things to make from said items, and probably the coolest creature ever.
Dubbed the "Wither Boss," this new baddie is a three-headed floating skeleton torso that shoots exploding skulls from each head. It does not appear as though the creature actually exists anywhere in game yet, but the files are all there, waiting to be hacked in if you've got some kind of death wish (as seen in the demonstration video above by Sethbling).
This update also adds potatoes and carrots, the former a reagent for baked potatoes and the latter a key ingredient in invisibility potions, of all things. Splashing the potion will make you and any surrounding mobs invisible, but will not affect your armor or any items you happen to be carrying. Meanwhile, splashing a baked potato on yourself would just be silly.