Mojang was released the first snapshot from the development of 1.6.
This snapshot in bullet point form:
New Minecraft Launcher:
Dinnerbone and Grum have been hard at work creating a NEW Minecraft launcher, with some pretty sweet features! To get the snapshot, use the new launcher available in the link below. To return to 1.5.1, simply start the old launcher and play as normal. That’s all there is to it!
Credits: The Minecraft and Minecraft Realms teams