Minecraft 1.6, dubbed "The Horse Update", has finally been given a specific release date. Mojang has announced the game will be released July 1st, and requires the new launcher (which is available in the source link below) to be installed. It's the same launcher that has been used for recent snapshots, so if you already have it you are good to go.Also today a Pre-release for 1.6 was released for the new launcher, which fixes dozens of bugs in the last snapshot. Fixes include hitting mobs while on a horse, health resets on login, spamming server logs with an anvil, redstone team names, and a few others. You can get the prerelease from the new launcher, or if you don't have it, you can download it from Mojang at the link below.
Source: Mojang