I'm seriously considering upgrading from the D60. The 3100 does seem to offer quite alot more. On the other hand I may turn to the dark side and get a Canon 550D, which offers lots more, but looks horrible, is big and heavy and won't take my filters or my beloved 35mm lens.
D90 is heavier than 550D (I've been considering the 550D partly because its lighter.)If you're looking at moving to a 550D, why not consider a D90 or D5000?
I wouldn't move to canon if you've got nikon lenses.