My recommendation for first time DSLR user:
D3100 is a good start for beginners in photography who do not wish to carry a heavy setup anywhere in the near future and don't wish to get too serious into photography within 3 yrs timespan (2 minimum).
D90 or D7000 are for serious beginners who wish to get into photography with a head start due to it's convenient and useful features such as bracketing and easy to reach settings on camera instead of in-menu.
I would recommend D3100 to ladies who wants to try DSLR to replace their point and shoot. If you have prior photography experience and understand the basic 3 elements (Aperture, Shutter speed & ISO), go straight for D7000.
For guys, I would recommend getting a 2nd hand D90 or a new local set D7000. D3100 is simply too small for guy's hand grip, imho...