You might enjoy reading about Haiku. Haiku is a code name for a project at Microsoft to make the ultra moblile PCs even smaller, longer battery life, etc.
Haiku was announced a few weeks ago at WinHEC. It’s a longterm project, post-Longhorn time frame, but similar to what you describe. There are several articles about it. (Google: Microsoft Haiku)
You bring up a great point about moving data between 2 or more PCs. How would you want it to work? Keep in mind that you’re moving between 2 machines of the same OS, not Windows Mobile to Windows XP Tablet PC Edition / Longhorn.