The rumored D90 replacement may turn out to be a 14mp camera after all, if it uses the same sensor as the D3100, or else it may use a Sony 16mp sensor. I can't quite believe that Nikon would have another APS size sensor with a higher resolution ready.
If Nikon is now taking more control of the sensor design in the cheaper models (previously they only designed the sensors in the professional models, starting from D1 to D3 series); then it means that some of the performance features of the professional sensors may trickle down to the cheaper models.
The other interesting aspect of the D3100 is that for the first time a model at this level is actually using the EXPEED processor. The D3000 model does not appear to use the EXPEED processor, although it may use the same software. Perhaps the new EXPEED processor is necessary to support full HD video, and therefore Nikon had to use it in the D3100. The good thing is that the with larger volume of sales, it should provide greater economies of scale and reduce cost.