Additional info from my experience:
I've transferred BTC off of TradeHill twice (funded my account with $$ yesterday.)
They claim it can take up to 24 hours, but in both of my instances it took less than two hours.
I would be happier if it were immediate and I hope that it will be...it seems to me that it is kind of in the 'spirit' of BTC to have it be so. But if I were running the operations and the systems were as new as TradeHill's likely are, I would start out with a semi-manual and/or human checked method of transferring them out.
I hope that when TradeHill achieves good confidence in their systems they can make BTC transfers out more immediate, or at least only whatever delay might be in order to avoid automated exchange arbitrage or whatever. I don't really understand the node reduction algorithms used to trim the block sizes down, but I cannot help but wonder if the bitcoin model could be harmed by an extreme number or automated transactions.