Hmm, reading the news on the bukkit website, it's a bit unclear when it'll be released - they seem to be waiting for official blessing from Mojang and/or some licensing faffing.
However, there is apparently
buildable source code available.
Hmmm. I guess some of the delay is because it looks like they may have decompiled the minecraft server in order to build it.
If anyone were brave enough to convert the alphabetized variable names into sensible things, it might even be that we could create our own customised server that could do whatever we want (within the game/client mechanics, anyway), including authenticating against a remote MySQL database.
Of course, having a proper API with Player and MySQL stuff all pre-done would be simpler, but with that not yet available, this does seem a possible tunnel to dig.