The new board has an automatic power select between the USB port or the DC Power plug.
According to David Mellis, the Duemilanove will be replacing the Diecimila.
I just bought a bunch of Duemilanoves so I can sell them on Liquidware. They aren't in stock at the current time, but I will be accepting back orders. The last I checked they were already in the mail on the way to my apartment.