Link: Nokia - Nokia 770 Features.
The Nokia 770 Internet Tablet's software is upgradeable and currently runs on the Linux-based Internet Tablet 2005 software edition. There is a planned launch next year of an operating system upgrade – the Internet Tablet 2006 software addition – that will support additional services, including Internet telephony (VoIP) and Instant Messaging.
I just found out that the skins, icons and graphics are all available under Creative Commons license. Link