Essentially, this is what happend to us, MtGox, CampBX, Crypto and to TradeHill
I think the long-run solution might be using Dwolla only for withdrawals (from exchange to my bank account) and inter-exchange transactions (from my mtgox account to my campbx account).
If you need to get money into an exchange from anyplace other than an exchange, you're probably going to have to do a cash deposit.
The situation above is the only way I can see Dwolla being able to make a profit on $0.25/transaction, after factoring in the cost of fraud. The fraudsters will keep getting more sophisticated.