The D3100 has a very good noise profile; it matches (and perhaps bests) the current leader in the budget dSLR category, the Pentax K-x. It produces exceptionally clean JPEGs up through ISO 800 and very usable ones through ISO 3200. (Adobe Camera Raw doesn't yet support the D3100 and Nikon Capture NX 2 doesn't provide sufficiently granular noise reduction for me to do raw versus JPEG comparisons).
Normally, ISO 800 images of cat fur are useless because the fine, dark fur is highly susceptible to color noise. But these are very clean, and I'll probably be able to work with the exposure and sharpness once I have raw software support.