When I'm typing like mad during NaNoWriMo, my keyboard is in my lap. That way my arms are at rest, and I just have to hold up my wrists ... I guess. Also, when playing FPS games I have the keyboard at the edge of my lap, and the trackball (thumb trackball, thank you very much) on my lap but closer in.
I find the average desk height to be wrong for serious typing, although for manipulating menus (Alt+N, ESC, and that's about it) that it's easier if the keyboard isn't restricting my posture...but that does mean I'm sorta slouched over the chair, or the desk -- whichever's easier.
Now that I have a working ergodox, I find that laptop typing is much harder, although with a short length of 1x6 I can still type my novels in my conventional arrangement.
In short: I'm actually really surprised lap-typing isn't more common. Its just so comfortable ... once you make sure the keyboard isn't going anywhere! I'd never own a laptop for that reason. Too hard to type on it, all the way around.