DxOMark score: D5100 vs D7000 vs D3100
DxOMark just published their final score for Nikon D5100. Although some minor differences, we clearly see that D5100 sensor performance is similar to D7000 ( overall score: 80) and way better than the Nikon D3100 (67).
DxOMark focuses only on image quality performance but does not cover many other criteria that are important when looking for cameras or lenses that fit your needs.