I doubt they'll raid people that are mining or just use bitcoin occasionally for legal things, or make you surrender your bitcoins or anything like that. Here's what I bet will happen:

1. Dwolla will suspend MtGox's account from their service, possibly freeze bitcoin user's accounts.
2. MtGox will have its domain name seized by DHS.
3. MtGox will be put under pressure to be shut down completely.
4. Bitcoin price takes a dip triggering a sell-off (hopefully won't be too bad).