I'm guessing because rake is typically lower at bitcoin poker sites that they don't offer any kind of rakeback?
BetCoin offer 2.5% rakeback on No Limit cash games. Nitrogen Sports have a loyalty program but no rakeback. Seals With Clubs offer rakeback ranging from 1% to 50%.
i don't know about bovada but i know that betonline doesn't offer any kind of a vip program or rakeback. i'm pretty sure that americas cardroom doesn't offer some kind of rakeback of about 25% but maybe someone can confirm that for me.
Ignition Poker do have some kind of rakeback in the form of poker points. For every 200 points you collect playing cash games, you can redeem $1 as an entry to a tournament etc.