Didn't Mojang hire some of the Bukkit guys?
Yep, most of the original creators, I think. They're the ones working on the modding API, at least theoretically.
Yes, they hired four of the main bukkit coders.
Of those four, only one is still involved with Bukkit as it was his project to begin with (EvilSeph). Seph appears to be the one leading the API work.
Dinnerbone is another, and he's actively involved in working on all aspects of Minecraft.
That being said, bukkit is still a separate project, has to be a separate project as it's open source, and still has to do the same procedure each time there is a new release (for legal reasons): Get the new binary, reverse compile obfuscated source code, make it work, then add in their hooks.
The snapshots was probably one of the first things that they implemented upon being hired.