I just checked my trades page before bed, and...
http://puu.sh/250De
Autobump is enabled, but the trade was last bumped six hours ago. Lolwut?
Both of my current open trades were "6 hours ago," and I could manually bump one of them just fine. Not gonna bump the other, I'll use it to see when autobump comes back.
Slightly confused here, but I'm hoping it's only a temporary bug.