I used to hold my N810 with my middle and ring finger partly curved and pushing their points into the back of the N810, my index fingers close to the top of the N810 (or the top of the back, if the device was open), and my pinky fingers sort of acting as a slight base for the bottom of the N810 to rest upon (but not a lot of weight on them -- they were more preventing incidental downward movement of the N810 to keep it form slipping out of my hands; the weight (what little there is) was largely supported by my palms along the sides of the back, and the sides of the device, pressing in a little). Which is also fairly similar to how I hold my G1.

But I never had discomfort with my thumb reach on it. My discomfort was from the fact that the keys were too stiff, and didn't have a decent tactile feedback/feel for when you had successfully pushed a key. Which is odd, because Nokia qwerty phones seem to get that part right (the E61i was decent, and the E71 is excellent, in this regard; but the N810 is awful in this regard).