There are more exchanges than 2, you know.   Heck, just go bitcoins -> namecoins -> bitcoins and you're laundered.

I believe this was possible because in the US EFTs can be reversed in case of error for up to 60 days even though funds clear in 2-5.  This is how it would go down, with no risk to scammer:

Joe Smith: "Mr bank, please initiate EFT to account XYZ."
Bank: K.
Dwolla: Funds received to Mr. Bogus T. Fradulent's account.
Joe Smith aka Bogus T Fradulent: "Mr Dwolla, please send em to Trade Hill, account ABC "
Dwolla: K.
Trade Hill: "Funds received to Bogus T Fradulent's account."
Joe Smith aka Bogus T Fradulent: "BUY BUY BUY SEND SEND SEND"
Joe Smith: "Mr bank, I made an error.  I sent to account XYZ, I meant PDQ."
Bank: K.  EFT reversed, reason of Joe Smith's funds deposited to wrong account.  Dwolla, never mind that deposit to account XYZ.
Dwolla: K.  Um.  Wait, what?  This sucks, let's just pretend we never transfered those funds to Trade Hill.
Trade Hill: WHAT IN THE BUTT?!?!

Kids, don't try this at home.  Not being an expert in wire fraud and money laundering I'm sure I left some important details out.  You're more likely to win 15 years of prison rape for wire fraud as opposed to a few bitcoins following the above script yourself.