Last week there was an Exim 0day flaw found in the wild. This hasn't happened to something this widely used in quite a long time. It's worth pointing out that all the right folks came together to get this fixed in an amazing amount of time. They did a great job and deserve a lot of credit. This could have been a lot worse than it was.
Upstream sent this message giving a pretty good run down of events.
Their openness is certainly the best way to have handled this. If you treat security like a PR problem, it becomes a PR problem.
The short story is that on December 7, a vigilant sysadmin (Sergey Kononenko) noticed a compromised server, and luckily grabbed a dump of the data. It wasn't widely noticed for about two days. During this time investigation began. Here is where open source showed its real power. When the folks investigating were having issues, they started asking other community folks to help, this eventually made its way to various vendors. Everyone brought a different piece to the puzzle, and the next day the problem was understood, and vendors started to patch their copies of Exim. It turned out upstream had fixed the issue quite some time ago.
It's not uncommon for emergencies to go horribly wrong, but when the right people do the right things, things can work nicely.
Calling it "0day" is a little sensationalist. The latest relase of Exim is not remotely vulnerable (as you note), but the distro maintainers didn't either update to it or back-port the security patches.
That said, I agree a lot of people behaved responsibly
Exim is a wonderful piece of software, and I hope its reputation isn't tarnished by the last week's events.