Mojang has released a preliminary copy of Minecraft 1.3 so that mod developers can get their mods up-to-date before the official release on August 1. (The Weekly Chunk: Minecraft 1.3 Prerelease
). The files can be treated like any weekly snapshot.
Mojang are also running a competition, asking people to develop a 30 sec video that uses MC 1.3 to promote/explain MC 1.3.
Our servers will stay with MC 1.2 until MC 1.3 is officially released and our Bukkit plugins are updated.