I get the whole Dwolla reversing charges after crediting TradeHill's account. But how does the scammer pull this off and how could Dwolla not be aware of it?
Dwolla is aware of it. But if they can just reverse their payment, they have no incentive to do anything about it.
The scammer transfers funds to Dwolla, transfers to TH, buys bitcoins, and reverses the original transaction from bank to Dwolla. Do I have that right?
Yes.
Is it assumed that the scammer stole access to the bank account?
Assumed by whom? Most likely, the reversal is the fraudulent part, but TradeHill has no way to know. That's between Dwolla and the person who reversed the ACH payment.