Possible, but if they don't have the funds available to back all their deposits, then that doesn't exactly bode well either.
They also allow withdrawal by check. They have no advance warning as to how much I will be withdrawing nor which method I will use to withdraw.
Though Dwolla is FDIC insured (or NCUA insured, depending on which pass-through account Dwolla has used for the funds) an exchange would probably wish to place customer funds in an interest-bearing insured account.
Because of delays in transfers, cash management can be quite a challenging task. Too bad there is no digital currency that is low-cost, non-repudiable and features fast clearing so that banks and corporations could use to transfer funds. A real pity it is.