It depends on how the site decides to handle it. There's BitCoin-only sites which will require you to buy BTC first to play with and get cashed out in BTC aswell, to then sell at mtgox.com etc. if you want. Seals-with-clubs is the most known BTC poker site afaik.
What a site like Lock could do however, is have the play remain completely in USD, but to process someone's cashout, buy BTC themselves with their own $, to then instantly transfer the BTC to their customer. No payment processor involved anymore (since this is something regular Lock support could do basically, "process" BTC transactions which is as simple as going to a BitCoin exchange site and buying some to then send them over to the BitCoin wallet/address their customer has given them. Regular support can't process USD payments however, because as per industry standard this should be done by a 3rd party payment processor also so that (on paper at least) the poker site doesn't have the financial data from their customers, but a 3rd party does. (and there's probably far, far more and more important reasons for this, but i don't know em and from a customer's point of view this is the most important one tbh
)