Credit Card authorization guarantees the funds to the merchant. 
Signing a bitcoin transaction and broadcasting it to the blockchain does not. Successful confirmation of the tx on the blockchain does, and to be on the safe side the rule of thumb is to wait for 6 confs on the bitcoin blockchain before considering the funds to be guaranteed.<p>Tx confirmation times are a known problem with BTC in terms of the payment acceptance flow, this is one of the hurdles preventing wider merchant adoption of BTC acceptance.