It's amazing to me that OpenX is trying to be looked at as a 'leader' in this space and they even do hosted ads, but they can't keep their servers up for a security patch release.
See my comment above - OpenX is being DDOS'ed. The upgrade didn't bring it down, though it's likely that the DDOS is intended to keep people from upgrading.
As someone said below, alternatives are really limited, guess that's the result of inexistent competition.