From what I know your bet will receive an instant response provided:
- all the inputs to your bet are confirmed
- you add a suitable fee to your transaction
The "advanced" part of the SD website says:
The best way for you to ensure your Bitcoin bet is returned to you quickly, is for you to include the standard fee of .0005btc in your payment transaction. This helps it get noticed by the network faster, and SatoshiDICE picks it up, calculates it, and returns it to you faster
0.0005 BTC is more than the required fee for lots of transactions, and way less than the required fee for others. I don't know whether the website is accurate or not, but if they instantly process all bets with a fee of 0.0005 or more then they're asking for trouble. I can construct a bet that requires a much bigger fee than that, and so will take a long time to confirm.
It would be good to hear the official position on how they decide which bets to process immediately, and to have it explicitly stated on the website.