When your withdrawal request is processed, the USD amount of the payout is exchanged, at a pretty fair conversion rate, for bitcoins, and these BTC are sent to the wallet address you specified. Note that the exchange rate that's used is calculated at the time the site actually sends your coins, not when you make the request. This spares you from some of the volatility involved in transacting with crypto-currency because of the short interval between when the poker room submits the transaction and when it is confirmed.
However, at Nitrogen Sports Poker, SwC, and blockchain.poker, there is no conversion to and from fiat currency because these sites do all their business in bitcoins, including the actual gameplay and all cashier functions.