Four developers from the Bukkit team will join Mojang. Their experience with modding and building tools for the the modding community will be put forward to build a proper modding API.
Mojang writes:
Many of you may ask why we decided to work with bukkit instead of other Minecraft teams, such as Spout or Forge. The reason is that we want more than just modding, and these guys have always had server admins in mind when developing their additions to the game. We hope that this will help the quality of Minecraft multi-player to improve, both for large and private family servers, while still being able to add fun stuff for the bigger audience.
Daniel Kaplan, who we had as a guest on Unfair Advantage episode 6, will be heading the project.