The only problem I see is that it takes 10 to 30 minutes for a transaction to be included in a block. If they give Bitcoins instantly could be double spent.
This is why SatoshiDice processes immediately, except for bets over 2-5 BTC.
If part of the bet transaction is doublespent there's a good chance it won't make it into a block. If the bet transaction doesn't make it into a block, the response transaction won't either and is therefore worthless.
If you broadcast a transaction and SatoshiDICE responds with a YouLoseTx, and you have built a block containing a double-spending transaction, if you "lose" you can broadcast your conflicting block. You wouldn't need much hashpower to do this and still remain positive. It's called a Finney Attack. https://en.bitcoin.it/wiki/Double-spending#Finney_attack
This is why they only immediately respond to smaller bets.
I don't find that scenario sensible at all. First buy $1000 machine, first customer comes in, buys bitcoins with $50, and notices that the machine does double spend. I believe he is not going to tell any of his friends...