You're absolutely correct. The following would completely work:

 1. Get ready to perform a [Finney attack](https://en.bitcoin.it/wiki/Double-spending#Finney_attack).
 2. Gamble some coins.
 3. If you lose, go through with the Finney attack. Otherwise collect your 1250 BTC from satoshidice.

Is anybody doing this? I don't know.