hiya rob,
I'm using an sl-6000 as my campus day-to-day computer, and I can keep well ahead of class notes on my infrared keyboard. I can't imaging being that fast on the small keyboard on the clamshells, not just because I'm used to the thumboard on the sl-6000, but because I used to be able to keep up in class on an HP 320LX, which, for those of you who didn't have the good fortune to be stuck with one, was one of the original palmtop greyscale LCD's with the first edition of windows CE. On to your questions:
slip in your pocket: yup, I sometimes keep mine in the inside of my jacket. very james bond.
touch typing: I use a microinnovations pocketop, like a lot of people here. it's supported by the irk driver, and although it's small, they keyboard is "cupped" so your fingers are stopped from going too far. aluminum shell, very worth finding. mine was $3 brand new on ebay.
wifi: clamshells only have wifi as a CF card, the people with clamshells seem to love this, and power to them. the 6000 has it built in, and that means I don't have to play around with CF cards when I move around. I like this better.
ethernet: all I can tell ya is that you'll need to ask around here for a supported ethernet cf card. no experience there.
windows sync: the word "easily" concerns me. word on the street is that they "do" sync with windows. I couldn't tell you. all I can vouch for is that they don't sync with Linux
Internet: yup, 640x480 is surprisingly satisfactory. I never get anything done anymore.
pdf's: qpdf2, load it up and drag around the page on the touch screen.
mp3: yes
word files: yes, but don't let it bring you down
instant message: gaim will talk to anything, that's what I use