Not a sideways update if one wants better high ISO. The D5100 will be about two+ stops better than the D3000. E.g. if you're comfortable shooting the D3000 at ISO 400, you'll shoot ISO 1600 on the D5100.
Of course the real question here isn't why upgrade to the D5100 or D7000 or even a used D300. It's -- what's the reason for upgrading in first place?