I have the Dwolla payment method installed and I it works OK when I don't have the One Page Checkout enabled, but with OPC enabled it does not show the box to enter the account number.
I realize you may not have any experience with Dwolla, but the Dwolla thread will have no experience with OPC. I will also post this there.
I am trying to find how to adjust this but I am not the best at php code let alone java, ajax, flash stuff.