Im not a nikon guy but have been looking into them recently. I think I heard people had some issues with the 3100 and auto focus, like there wasn't a motor, and if you want autofocus, you need special lenses which in the end will limit you.
The d90 is really nice, takes awesome hd video and you can get a rly nice used one here for 649 bucks.
http://www.usedcameras.com/SearchRes...show=10&page=5
new ones i think are like 799 bucks
this guy uses a d90 for his hd stuff, check out some of his videos. obviously hes done some post on the movies but its still a great example of what you can do.
http://vimeo.com/22080133
here are some image examples full res
http://www.pixel-peeper.com/cameras/?camera=1039