The Minecraft API is going to make mod development SO much easier. No more decompiling and patching hacks. I've been waiting for this since Mojang snagged the Bukkit team. I look forward to the first client/server builds that support it.
Bukkit made it really far with community extensibility via non-official APIs. Now they get to help define the official API based on real-word experience about what developers want to do with customizing Minecraft. They also recently announced that they are merging single player and multi-player, creating a single mod-able codebase.
http://www.mojang.com/2012/07/minecraft-1-3-info-snapshot-w2...
I added a Jenkins build job for it on BuildHive so anyone can download the bits if you want to play with the very limited banning API that they apparently have so far :) https://buildhive.cloudbees.com/job/swashbuck1r/job/Minecraf...
I hope they'll consider adding a build job for the official project.