However, since it is clear that Dwolla transactions are in fact reversible (contrary to their claims), exchanges like TradeHill might need to change how they process Dwolla deposits. To protect themselves, they may have to start withholding bitcoins purchased with dwolla deposits for a couple of weeks, until it is clear the deposit won't be reversed.
Sounds like that won't be very practical:
...in the US EFTs can be reversed in case of error for up to 60 days
Having to wait for two months before you are actually allowed to use your deposited funds to buy Bitcoins is surely not an option.
Dwolla probably didn't have that many scamming issues they couldn't resolve or just eat the loss up until now. I'm not sure about the US, but EFT reversal is not a very common scamming-vector here in Europe.
I suspect that the only real way to handle this is for somebody to charge higher fees and use that money to pay for lawyers handling the dispute resolution / scamming cases.
Whether Dwolla, TradeHill or the bank does that is irrelevant but somebody will eventually have to price in that risk.