I ordered IBM's new Thinkpad X41T convertible...reviews of it are pretty good but I don't expect to see mine for like a month because IBM is in serious undersupply. I'd say it's pretty much the ideal choice for someone who wants a tablet without completely giving up the ability to type. Of course, nothing stops you from using a USB keyboard with a tablet.
Outside of that there is HP's TC4200, it's similar, slightly inferior build quality in screen, slightly heavier, and slightly faster and more upgradable. HP also has a pure table(tc1200 or something) for around the same price which has a dockable keyboard but not permanently attached one.
Then there's the Toshiba Tecra M4, which is more of a laptop with a tablet screen just useful for dabbling in it. It's heavier than the others, but also very powerful, and rather poor build quality for a tablet. Then Toshiba also has the M200, which is more of a traditional convertible.(toshiba has a few more tablets besides that too) The screen on the M4 is poor but high resolution, whereas most are much lower res. There's also the protege, and acer has some. Acer even has a sub $1000 tablet, but I don't think it's light wait or powerful.(well, it's better than the transmeta and VIA based tablets, it uses an athlon xp cpu)
As far as pure tablets, Motion's are supposed to be pretty good, they just released their LS800. I believe it's rated very similar to the X41T, except smaller screen(8.4 inch compared to 12"), no keyboard, and even lighter than the already very light X41T. X41T is 3.5 pounds with 4 cell battery and 4 pounds with 8 cell battery, whereas I believe the LS800 is 2.5 pounds.(obviously if you're writing on something you're holding in your hands, you want it light.
Check out this site:
http://tabletpcbuzz.com/ and read some reviews or something.