The D3100 is the second successor to the D60 along with the D5100.
Whatever D60 had or could, the D3100 have and does - and some more too i.e video - it's a newer generation.
You can compare them here in DPR.
The biggest differences, I think, is the sensor - D60 had a CCD-sensor, from what I read here, a lot of people miss, D3100 a CMOS and some higher resolution. It made LiveView possible for whatever that Counts, and video too.
They are both entry-level cameras with a lot of capabilities.