I have my D300s & it's more than enough camera for this geezer to digest for the time being, if at some point down the road I feel the need to upgrade I just might, but holding the line at this time.
Personally, the only attractive feature over the D3100 is the sensor, but not enough to upgrade. The flip screen does nothing for me except make it larger than the D3100 which is a negative, I like the compact nature of the D3100. HDR feature is half baked along with very limited Bracketing options. Unless an upgrade to the D90 comes out, I'll get a D7000 in a few years.
I have my D300s & its more than enough camera for this geezer to digest for the time being, if at some point down the road I feel the need to upgrade I just might, but holding the line at this time.
If I was to upgrade, it would be to a D300, not any of the newer ones. I don't like video in any of the new ones, I like a focus motor built into a camera so AF and AF-S lenses can be used, and it is more camera than the D5100 will ever be, IMHO. Then again, I'm old and opinionated.
I'm young and opinionated LOL well young(ish) I think that the D2x still does what it does at least as well as the new D5100 so I'll be keeping my brick for a while longer yet, if I was to upgrade I think that it would be a D3 (if I had the cash) but if I want video then I have a video camera. I think that the D5100 sounds like a great entry level DSLR and Nikon has been very good at those in the past so I hope it keeps up the tradition.