It seems like LR2 is applying noise reduction to the <100% magnifications... I'm not sure what was happening because after testing exports of the same image with LR1 and LR2, the output seems identical.
Very simply, they have probably gone from using a very poor method of downsizing on screen, to a better one. Adobe downsizing on-screen has historically been *HORRENDOUS*, and has fed the myth that higher pixel density necessarily yields to more image noise.
I don't have LR, but I have CS2, and in CS2 it is very easy to demonstrate how poor the <100% zoom is. Make a canvas of 50% grey, about 400x400 pixels, and add 20% gaussian chromatic noise to it. Duplicate the canvas a few times, and zoom each one to a different amount, say 100%,50%, 25%, 13%, and you'll see (in CS2), the same visual noise in each. Now, duplicate the same amounts again, and this time, downsample with bicubic by the same amounts as the zooms you used, and then view all of these at 100% zoom. The ones downsampled the most should have the least amount of visible noise. When noise remains the same at the visual pixel level with downsizing, the method used is not only increasing apparent image noise, but is also severely aliasing the image as well (as CS2 also does).