Nikos, the D3100 is nothing like the D700, it is an entry level camera designed for beginners. Since the camera isn't even out we have no idea how well it will perform ISO wise, but I expect that it will be at least as good as the D90/D5000, if not a little better. The D3100, like the D40, D60 and D5000 can use AI-S, pre-AI lenses, but it will not meter with them and you'll have to guess the exposure.
Niko, I think the D3100 is Nikon's answer to the EVIL cameras, especially now that Nikon has a nice set of AF-S prime lenses with the introduction of the 85mm F1.4G. If the ISO range proves to be good, just think about it, with some serious cash you could have a nice compact camera body, and use the AF-S prime lens lineup, 24mm F1.4G, 35mm. F1.8G, 50mm F1.4G and 85mm F1.4G or 85mm F3.5G VR for more cash strapped users. That may be a lot of money, but if size and weight are an issue that is a solution, as none of the primes are overly large or heavy.