The D3100 is a big improvement over the D3000. Its got a 14mp CMOS sensor, whereas the D3000 has a 10.2mp CCD sensor, which is a bit dated now. It also has 1080P video WITH autofocus. Even the D90 doesn't have auto focus on the video! Maybe not a major selling point before but the included autofocus makes it great as a video camera.
The D3100 has live view, which if memory serves, the D3000 doesn't. It also has automatic chromatic abberation correction, something which was available on the D5000 and D90. Still no onboard motor though, so you are stuck with AF-S lenses or manual focussing D lenses.
Out of the 3 (D3000, D3100, D5000) The D3100 would get my vote.
Go for the Nikon. its from a a stable that has years of experience in photography, where "Sony" is only a"Jonny come lately" to photography. although the do make Great T.V.s ,DVDs and so on they have not inpresed me
It's only a matter of time before Sony catch up. They have the A55 which is a blinding piece of kit. 10fps and continuous auto focus throughout high speed shooting and also very fast AF during HD video. It won't be too long before they get an increased market share, with all of the money they are ploughing into their cameras and ads. I remember someone saying "Don't get a Sony, they will drop out of the DSLR market, Have they? No, they continue to invest heavily.