As jk-menifee mentioned, the account user details are:
username: ubuntu
password: marvell

It's actually on a small piece of paper in the box that the D2Plug comes in.

Anyway, has anyone else had a good play with their D2Plug? I'm struggling to get WiFi working on it. I managed to set WiFi settings in the Networking section, but WiFi doesn't appear in the top right hand corner under selectable network options. Also, "iwconfig" doesn't seem to show any WiFi devices.