I had a Panasonic Toughbook years ago with a 12" screen, and then I got a Dell a with a 14" And although the 12" didn't really bug me as I remember, I did appreciate the extra room that the 14" game me.
But I'm debating about this too. Should I get a 12" iMac or Powerbook, or a larger model? Ultimately I think the weight difference is perhaps less than it seems -- probably only one or two pounds. So for that is it worth the smaller screen (it's a fair bit smaller when you look at them side to side).
I'm leaning strongly towards the 14" to be honest. Because it is just a lot easier to compose documents with a bigger screen. Your eyes don't get as tired and you can see more of the document on your screen (and have a better idea of what it'll look like when printed).
The other thing you might want to think about is the fact that the 12" IBM doesn't have a touchpad. Now I admit they have the best eraser mouse I've seen, but I still don't think it's anywhere near as good as a touchpad.
Well the X40 is 2.7lbs and the T42 is 4.9. I wouldn't think that 2.2 lbs would make that much of a difference, but to me it did. Also, as someone said, the resolution kinda makese up for the small screen with the X40 since, if my eyes get that tired, I could enlarge the writing. But you're right about not being able to see the entire document with a smaller screen, but I don't think that will bug me as much.
The salesperson showing me the laptops mentioned that a lot of people who are accustomed to touchpads don't like the X40's because of that eraser mouse, since it's hard to get used to. However, since I've never had a laptop and am used to a mouse, perhaps the touchpad would be equally hard for me to get used to. When I was comparing them in the store, both felt super awkward to me.
Ugh, decisions decisions. I just love the fact that I won't have to break my back with a smaller laptop since I intend to carry that thing EVERYWHERE. Plus, the X40 takes up less amount of space than the T42, which would help when you're surrounded by books, outlines, etc. at your desk.
Damn eyes. At least you guys who've had 12" laptops have a reference regarding what your eyes can take. :::sigh::: It's just the SIZE...