Love the D40 and think I will continue to use it often, though right now I am obsessed with my new D700. AMAZING camera.
I think the person who said, wait until you run into something you really cannot do with the D40 and find a camera that can was on the right track.
My main motivation is that I do a lot of low light photography and the D40, though sometimes making a great shot under these conditions, more often than not did not quite make it. Noise at ISO 1600 is also too much. The D700 excels under these conditions. You almost cannot make a bad shot in poor light, even the armrests of my grandmothers chairs look like works of art :-0.
Otherwise the D40 has served me wonderfully and has shown me what is possible with a DSLR and has freed me up to be creative and spontaneous with photography without thinking too much about the technology.
With the D700 I do need to think about the technology more so and have a lot to learn. But I think once I have learned, my freedom to express my creativity can return.