What follows in a thought experiment: what if the new Resume feature in OS X Lion made it ridiculously easier to have a portable home directory?
For those not familiar with Resume, it allows you to "restart your Mac and return to what you were doing — with all your apps in the exact places you left them". Presumably, this would also work the same way if you logged out and back in.
So here's the thought experiment. Imagine you had a 13" MacBook Air, which as a development machine isn't that great when compared to a super fast desktop, but it's still pretty good and you like to use it in coffee shops. Now imagine that Air also had a Thunderbolt port on it and you could use the SSD on the Air as a home directory for your account on the desktop machine.
At your house you are working on the Air where you're using Safari and you have an untitled TextEdit file open with some text selected. You quickly log out, close the Air, plug that into your desktop machine (via Thunderbolt), log in and up comes Safari and TextEdit with your untitled document all ready to go- even with the text you had selected. You open up Acorn to do a little bit of image editing as well. All your data is still sitting on the Air.
Eventually it's time to head to the office. You log out from your desktop, unplug the Air and bring it with you. When you get to your work computer you plug the Air into your desktop machine, log in (once again, with your home directory on the MacBook Air via Thunderbolt) and continue working exactly where you left off. Except, maybe Acorn isn't installed on this machine? But that's easy enough- you just open up the App Store, click a couple of buttons, and Acorn is installed. You're working again with very little fuss.
Something similar to this has been possible for a long time using networked home directories. But the network is slow and unreliable while SSDs and Thunderbolt are super fast. We just have to wait for Thunderbolt enabled Airs and desktops.
At any rate, I think it's a neat idea and it might catch on for a certain class of users.