Every time new equipment gets announced, especially a camera with promising new features or abilities--my In Box starts filling up with the "should I upgrade" questions.
The answer is quite simple: you shouldn't upgrade unless you've exhausted an important ability of your current camera and know that the new one will clearly surpass it in that ability. As we get deeper and deeper into the digital era, each generation of new cameras tends to produce smaller and smaller improvements, or improvements that are difficult to distinguish in "normal" prints and use.
As poor as the camera companies are at marketing overall, NikonUSA's press release on the D7000 used the words "exceptional," "unprecedented," and "stunning" just in the first two sentences. Add "innovative," "revolutionary," "enhanced," "groundbreaking," "amazing," and "striking" in the second paragraph, and you can start to see how people might get the idea that the D7000 might be something they need to look into.
Don't get me wrong, the D7000 looks like it might be a real winner, specification-wise. But since I don't yet have one in hand and won't until Nikon actually ships the product to customers, I can't yet make any statements about image quality and performance, and those are generally the things you need to know about in order to make a considered decision rather than an emotional one. That said, as the next article notes, if you want a D7000 in the next two months, you'd better have already gotten in the queue at your local dealer.
But what's happening right now on the Internet is mostly subjective hyperbole, as usually happens prior to a new release. Just how good the D7000 is or isn't is still unknown. If you've been waiting for awhile to upgrade from a D40, D50, D70, or D70s, I've no doubt at all that a D7000 is a big step forward. But if you're thinking about ditching your D90 or D300 for a D7000 (or getting a D7000 instead of a D700, as some people seem to be expecting that the early hype that the D7000 was as good in low light as a D700 is true, when it most likely is not), the jury is still out.
Meanwhile, I can say a few things about the D3100, since I've been testing it since arriving home from Africa. For still images, I'm quite impressed. Despite the push to a new 14mp sensor, it seems to beat the old 12mp sensor cameras in my low light tests (but only by a small amount). Someone thinking about moving from a D40 or D50 to a D3100 would be moving quite a ways forward in terms of still image picture quality. Those moving from a D40x, D60, or D3000, would still be moving forward (especially at critical higher ISOs like 1600 or 3200), but not particularly far in terms of resolution. The video side, however, leaves much to be desired. I'll have much more in my upcoming review.