They won't outlaw Mt gox. That makesn o sense. They woll outlaw trade and possession of bitcoin because of money laundry and being an alternative currency. US citizens are required to use the USD. It is illegal right now.
Could you cite your sources please?
Regarding required use, I could not find it quickly but I am pretty sure no second currency is allowed.
Regarding laundry, look at
http://bitcoinlaundry.com/. I steal money from a bank, buy bitcoins, launder them and cash out to USD. Easy.
US dollars are legal currency. What this means is that if someone wants to repay a debt and they offer you US dollars to do it, you are legally required to accept them. However, if it is not a debt, but just an actual transaction (buying groceries for example), they are not legally required to accept USD.
Second currencies are allowed, but no one uses them for many reasons, Greshams law being one of them, but also because it is just impractical as so few people are generally willing to accept them.
As for laundering money, you are misunderstanding how money laundering works.
Using your example of a bank robber who wanted to launder his funds, what he would have to do is funnel them into a cash business as extra revenue. They would then come out on the income statement as ordinary income, which he could then pay taxes on, and viola, laundered money.
If someone robs a bank, then buys bitcoins and launders then, then turns them back into USD, he has accomplished nothing except wasting transaction fees, as he still has a pile of cash.