I really like this concept, and, perhaps oddly, I like it even more if the subnotebook is actually a leach of sorts on a more powerful CPU. In other words, if I could use my regular Mac - a decent iMac, a Mac Pro, etc., with a subnotebook in the monitor constantly being kept up-to-date with everything in my home folder, then I could pop out the subnotebook, take it wherever, keep working on my files, and have them automatically synched when I return.
Done properly, it'd all be seemless, and I could use Back to My Mac to fetch larger files stored elsewhere.
And it would overcome the issues of a subnotebook having inferior capabilities - a dock with an HD is nice, but a whole other, faster Mac is even better.