While we do a vBulletin installation upgrade.
If you see this message beforehand, then you won't be surprised.
If you're seeing it later, after the site is back up, then now you know what happened.
If I can get an exact time of the outage (this isn't something we do in house, it's done by our hosting provider) I'll post that here as well.
UPDATE: We've scheduled the software install for just after midnight, CDT. I look forward to waking up to carnage and mayhem tomorrow...