A laptop on a keyring, well in many ways this can only an exaggeration, but the idea has been intriguing me recently. The catalyst was coming across Knoppix.
There's a lot of hardware around out there, and at times it seems it would be easier to just go up a computer and use it than to lug one around all the time. But even if there were lots of computers around, it's still too hard. The problem isn't your data, networks can schlep that around, and USB memory devices are getting big enough to easily hold a working set.
The problem is applications. I've been on a mild spree of OS experimentation with a new Intel desktop at home (for Windows XP and Linux), a new Mac laptop, and a new Intel laptop "any day now". Any time you get a new computer there's always time to spent training it, meaning installing all the apps you need and setting up the configuration. In many cases the advantage of the laptop over a random desktop is the presence of my application environment.
Knoppix, and in particular its customizations, offer another route. Burn the OS, all the apps you need, and your static configuration stuff onto a CD. Then when you get to your hotel, walk up to the in-room PC, insert your CD and your USB fob for your data, and you have all you need. No need to tote around that laptop. As these keyring USB drives get bigger, that may even be enough on its own (I installed a reasonably functional Knoppix customization in around 500MB, which is within the range of SD and compact flash cards at the moment.).
Now this doesn't solve all laptop needs. I often use mine on planes and in other places where I don't expect to see computers easily offered. But it may open up space for other devices to join in. Many years ago I got sick of the heavy slow laptop I had in those days. I decided to get an NEC MobilePro 700. This was a tiny 'knee-top' with a small but usable keyboard and a half-height screen. While it was too big for a shirt pocket it only weighed around a pound, easily opened even in coach, and ran for hours on 2 AA batteries. For email and light writing it worked pretty well.
A modern equivalent of something like this might be the Sharp Zaurus SL series. A PDA with a screen, tolerable keyboard entry, WiFi, and decently full operating system could handle those times when you really need something on your knee. You then use your CD and keyring when you have a bigger box available for more serious work.