Actually, the issue is not so simple. Technology is improving every year or every two years based on model releases. If the number of megapixels in the camera would have remained the same, then the quality of low light imagery would have been increased manifold.

For eg. when a new camera comes to the market, the sensor size remains the same. However, the number of Mega Pixels increases. So the size of each pixel decreases. Though the camera manufacturers come out with gapless pixels so that all the area of the sensor is used to gather light etc, tweaking the algorithm, better processing power of the chip etc it does impact on the ISO sensitivity.

So when I compared the 50D (15 MP with a 1.6 crop factor sensor) with my 1D Mark II (8.2 MP with a bigger 1.3 crop factor sensor), I realised that despite the noises made by lot of experts, the Mark II holds out.

Also, you should remember that in the newer cameras, you will find noise as well as fine details. In cases, we would prefer the fine detail despite the noise.

I wish they keep the entry models at 10-12 MP and give clean ISOs. The D3100 is a recently announced camera. It is the first Nikon camera to have 1920x1080p video capturing ability. It has 14.3 MP. I guess the image quality would be good. I haven't seen images to know about the high ISO ability. About the sensor there is guessing game going on. Some say that this is the same sensor as used in the Sony NEX cameras. If so, it would be definitely better than the D80/D200 in terms of lowlight ability.

Cheers,
Sabyasachi