All I would say is if you have any bitcoins or cash at MtGox; get it out now! You can't fight the US gov, you just have to stay out of their way.
I know, I know, ..I was thinking same thing after I posted it....
But in all seriousness, I think there's more to come. Dwolla is a legit US based company, so it's easy for the US gov to force their hand. I've never heard of the DOJ of just freezing 1 account. If they are serious, which I think they are, we will hear of more MtGox's accounts being frozen or websites saying they can't send payments to MtGox over the next week.