Minecraft version 1.3 is on the way, and this update looks to be a doozy. The most sweeping change will be a merging of single-player and multiplayer modes. More specifically, single-player as everyone knows it will now be a "shell on top of multiplayer."
The reason behind the change, according to Mojang, is to accommodate the upcoming modding API, which otherwise would have required modders to create multiple versions of each mod, one for single-player and one for multiplayer. The downside is that single-player games will now require a little more computing power, though Mojang is hoping to smooth things out a bit in 1.4.
Other additions in 1.3 will include emeralds and emerald ore, the ability to buy items from villagers, the ability to write in books, an optional "bonus chest" that helps new players "get started quicker" and more. Minecraft 1.3 is slated to arrive August 1, with a "release candidate" available a week prior.