I was browsing the internets and found this, of course only 12 hours after depositing at UltimateBet.
I recognize that it's 12-18 months old, and not technically "proved," but I found this to be fairly substantial evidence of cheaters at UltimateBet and Absolute Poker from a reliable source.
It's a long read, but I recommend it for anyone who has money at, or plans on depositing at, one of these sites.
Whether or not these players are still around is unknown to me, but it definitely makes you think.
For your viewing pleasure:
Superusers and Silence: How UltimateBet let players get cheated for millions - Poker News - News, Views and Gossip
Check out the Youtube video of a simulated $1000 buyin game from the hand history of one of the disputed players:
http://www.youtube.com/watch?v=FczbS7FiWSM
I'm not saying these sites are definitely involved, I doubt low limits are effected much, and hell, I've even got money myself in UB, but just be careful.