they might just have gotten cautious ... the longer they delay releasing the funds the better the chance that fraud will be detected while they still have control of the money.
+1
It's normally an instant process for them to be made aware of the transaction. If you look at your bank statement you will see that a transaction is listed as PENDING for several days. This usually means you can cancel the transaction. Dwolla should have never been transacting funds before transactions were fully approved for this very reason.
'Instant' transactions aren't instant, and when you get a 3rd party involved you should be suspicious of things speeding up. This is one reason why I have never bought into the Dwolla hype.