We've stumbled upon something far more interesting than a bet. Why on earth would you entertain the validity of forbidding alternative currency as a 'non-stupid' law?
There are cases in my mind where giving governments the ability to freeze assets makes sense to me. For instance, freezing all of Ghaddafi's assets. This wouldn't be possible with Bitcoins I believe. I am certainly aware there are benefits to have a means of exchange that isn't controlled by governments, but I'm not 100% convinced yet.
How about making the bet that BTC is worth it's prebreak price six months after a dollar break? So of its illegal tomorrow it's worth $10 dollars again by Jan 2012, as calculated converting from BTC to trading currency to USD.
I will take that bet if it includes that there is still no Dwolla/MtGox-like ACH way re-established within the 6 month period.