I've had a Motion Computing M1300 for seven months or so now, and I love it to pieces. I love the slate format, and the new Input Panel is 90%+ accurate recognizing my cursive "font" (one of three, and not my preferred one, but oh well). Six of these months I went completely keyboard-free, relying solely on the Input Panel. I was mostly successful, and while writing is slower than typing, I think I do more editing in my head when writing longhand than I do when typing, so in the end it doesn't end up being much slower.
Still, it gets cumbersome when I'm doing a lot of writing, and of course handwriting recognition is pretty much useless for writing code. So I bought a Half Keyboard, which is small and light enough I don't mind hauling it around everywhere I go. (Rory's idea for popout thumb keyboards is intriguing, but I think the Half Keyboard is better for continuous use.)
[I've looked at the convertible tablets -- my developer has a Toshiba Portege M200 -- but the keyboard/chassis adds just enough weight to make it cumbersome to hold.]
While I do take my tablet everywhere I go at work, I am much less inclined to do so elsewhere. It's just too darn big to easily tag along. While I do carry a "man-purse", it's only ~5"x7", and I'm not willing to use anything much bigger.
So the recent reports about the FlipStart PC caught my eye. Scoble says he "want[s] one, but would be more likely to spend the $1500 on a Tablet PC". Lora at WhatIsNew says "the feature that is glaringly absent is a digital pen. The FlipStart would make a great, sub-Tablet PC". OQO has a similar concept that exchanges the mini-keyboard for a touch pen (a la PDAs).
I drool over both of these, but I don't really want either -- they're too small. I never use my Pocket PC anymore for the same reason. What I really want is a Tablet PC with a screen that's about 4"x6" (about the size of a paperback book; note that the *screen* is 4"x6", so the Tablet itself will be a tad bigger), has a built in Half Keyboard, and comes with a laptop-size dock. This gives me a Tablet small enough to take everywhere I go, big enough to be usable in meetings, a mini keyboard usable over sustained periods, and when I really want a bigger screen I can just dock it into its laptop shell -- which itself may be hooked up to external monitors.
If your top secret project happens to be just exactly this, please let me know! <g/> (That email would be michhu at microsoft dot com.)