[Animats]

From the standpoint of a single transaction, Bitcoin is consistent eventually. It's certainly not consistent immediately. Only when the transaction has been accepted into the blockchain and many blocks built upon its block is it effectively irrevocable. Early in the transaction's life, it's still revocable.

(The people behind Satoshi Dice found this out the hard way. They accepted bets with zero confirmations. Someone realized they could cancel a losing bet after losing by double-spending. Oops.)