The money was seized by the DHS and probably placed into some sort of trust account (ironically, it may still be sitting at the same bank it was seized from). I don't think DHS ever claimed that was Mt.Gox's money; only that it was money being illegally transmitted by Mt.Gox for US customers.
I believe the funds have not been seized yet, only frozen. I know the media uses those terms interchangably. Frozen means the funds are still in the account but bank is prohibited by court order from allowing any deposits or withdrawals of any kind. You can get funds frozen with just a warrant. Seizure generally a civil lawsuit and verdict (or pre-trial settlement).
If DHS eventually does seize the funds it will be put into a fund where in theory victims will be able to file a claim. Any cost of managing that fund (which may run into the millions of USD) will be deducted from the fund before any proceeds are paid. As for getting the funds in a timely manner. I had funds @ Full Tilt Poker (along with millions of other victims totally $300M?) when their accounts (not in US banks BTW) were frozen were frozen. After FTP accepted a settlement with DOJ, the funds were seized as part of that settlement. That was 3? years ago. To my knowledge not a single penny has been returned yet.