Hm...interesting.
I came from a Treo 700p, and one of my big reservations for switching over to the iphone was the loss of a tactile QWERTY keyboard (I made the jump on 3G launch day 7/11) and IIRC, I even posted some reservations about it on treocentral...well, it's been almost 3 months later, and I've never looked back.
I've pretty adapted to the touchscreen. I used to wish that future iphones would include a pull out QWERTY keyboard (like the Instinct), but now, I think like that might ruin the "eleganant design" of the iphone. :shrugs: YMMV.
However, if they wanted to develop an EXTERNAL add-on keyboard as proposed by the original poster, that wouldn't bother me,
Brent