They told everyone, and continue to tell everyone, that they don't. I presume they're going to reimburse you, change their terms of service, and notify their customers. If not, they cannot be trusted to honor their commitments.

"Remember, these are cash-based transactions! No credit card fees, chargeback concerns, or signing necessary!" -- Dwolla

I bet they were thinking something like, "this is so easy, why isn't everyone doing it?" Now they know. It's never easy.

If this goes to arbitration, you should win. Dwolla can't excuse their reneging on their agreement with you because a third party reneged on them. Otherwise, you could excuse any non-performance simply by laundering a contract through a third-party who you would rely on to perform your part of the contract.