Just picked up a D3100 today. This camera is really a gem, Day 1 and I'm already sure that I've made the right choice. Was looking to upgrade from my D70, another great camera by Nikon. However, experience over the past few years has seen my Canon G10 seeing more action since it offers a more lightweight and portable option for day to day shooting than the D70.
Liked the D90, but that seems even bulkier and heavier than the D70. Didn't want to invest in a full-frame digital; as a "prosumer" who loves photography as a hobby, I pretty happy with the "compromise" of the smaller sensor of the DX series.
Pondered the D5000, was swayed by Rockwell's review of it, but it was just a little too lite on the features I enjoyed on the D70. Looked at the promising 4/3rd cameras, but felt the G10 was just as good as many/most of them.
Saw the D3100 announcement and that looked promising. Waited to see what D90 replacement would look like and felt too big and too expensive for what I was looking for. The D7000 is a marvelous looking camera on paper and I'm sure will make a lot of people happy.
However seeing the feature set on the D3100 ( http://www.infoborder.com/Digital_Cameras/Nikon/d3100.php
), and the weight and size, was pretty convinced that it was the route to go. After getting it today, am absolutely sure about it.