No, it was only open and public for an hour. It could have been open for months, maybe longer.
It was the result of a change today, right before it hit Hacker News (so sayeth Marco of tumblr in the #tumblrs irc channel, anyway; I believe him).
As we know, hackers regularly turn random door knobs to see which doors open. Logs i can see show more black hat attempts than white hat, so either OldGregg's friend got lucky or a few exploits might have already been made.