Not sure why all the comments here are harping on the "one line" thing. The point is that you can transfer money with one command and no pre-existing state. That's pretty great.
And no security. That's not so great. EDIT: What I mean by this is that if you give me your account number so that I can deposit into it, then I can also withdraw out of it (not through the Dwolla API, but trivially by other means -- see http://perimetergrid.com/wp/2008/01/01/checks-the-most-dange... )
By submitting a file straight to the networks, I could also debit a nonexistent account for $20m and have it show up in my account the next morning. Wouldn't hang around for long though.