This is, IMHO, not totally what I would call responsible disclosure.
From @notch: "We took down the auth servers until they've been fixed. I'll pass on everything I learn about what's going on, just woke up. #groggy" "Also, in the future, if hackers could please not find exploits in the middle of the night on weekends, that would be great, mk?"
I have heard of cases where informing the company of a vulnerability and telling them "I will publicly disclose the vulnerability in N days" has resulted in security researchers being taken to court as a "blackmail attempt". Annoyingly I can't find the case I'm thinking of (though I've found numerous other unsettling ones such as [1]) but will update this comment if I do. One example of such madness is Dmitry Sklyarov[2] who was arrested under the DMCA's anti-circumvention laws for revealing that an e-book vendor used ROT13 to encrypt their documents.
A useful introduction to the complexity of public and responsible disclosure can be seen at the EFF's Vulnerability Reporting FAQ[3].
[1]: http://www.scmagazine.com.au/News/276780,security-researcher...
[2]: http://en.wikipedia.org/wiki/Dmitry_Sklyarov
[3]: https://www.eff.org/issues/coders/vulnerability-reporting-fa...
Especially given the fact that Notch and Mojang have played Quake with Team Avo members.
A different set of blackhats "Team Nodus Griefing" had it before the gist was up and fell into a honeypot (with packet capture to find the exploit) set by Mojang's Bukkit team and the reddit mods: http://www.reddit.com/r/Minecraft/comments/wl0zy/psa_exploit...
So I don't think this was meant to be responsible disclosure, it was more laying out the facts to get some internet cred ;)
Props for the speedy fix though.
[0] http://www.mojang.com/2012/07/houston-we-have-a-problem/