Has it just disappeared?
Nope, it did not disappear.
But SatoshiDICE did everything correctly (the payouts went to an address used to pay for the wager). That's not a "normal" way to handle bitcoin transaction it but it is what it is and it works if you know what you are doing.
So likely some customer of Mt. Gox just received your payouts. Good for them. Bad for you.
Mt. Gox could easily prevent this from happening. If you were to try to use Bitcoin-Central to send a wager to SatoshiDICE their system would refuse the withdrawal request. They've gone through the effort of blocking those addresses for withdrawals, They did it as a service to their customers who might not know how that works. (There are dozens of gambling services now which do this same thing, so you could still make the mistake at Bitcoin-Central playing on some other gambling site, but not playing on SatoshiDICE.)
Now it is possible Mt. Gox could contact the customer whose account got credited with your winnings and see if that party will voluntarily part with their newfound wealth. Perhaps that amount has already been withdrawn by that customer and the funds are long gone.
This would be the same as forgetting a wad of cash in the drawer at your hotel room and then the next guest ends up with it. It would be right for that person to return it, but they have no idea how. In that instance the hotel's rental records might help them find the rightful owner. But if you were to contact the hotel and in turn the hotel checks with the guest but there was no cash found or the guest has already vacated, I doubt the hotel would do much more at that point.
[Edit: I do question SatoshiDICE on how hard it is to see the link to "Read this warning first" on their site. The warning is needed and something like that should show more prominently on their site, in my opinion. I don't know if that would have helped to have prevented this specific mistake from occurring, but the way it is now very few will notice it.]