KmH hit it dead on. I would use the 3100 until you come across a scenario that you can't work with (not enough fps, ISO capabilities) and then look for an upgrade that suits your needs. Personally, I use a D5000 but am hoping to upgrade to the D300s as I shoot a lot of sports.
i feel both posts are spot on. i just recently went from a D3000 to a D7000 myself. find out what is limiting you with the camera you have and then find the camera that takes care of that issue.