Baltimore Checkerspot. They prefer wet areas where their host plant (the turtlehead) lives, and thus they can be hard to locate and hard to sneak up on.
This past weekend I was hiking this morning at Madison's Cherokee Marsh conservation park with members of the Southern Wisconsin Butterfly Association. The park has a long boardwalk which traverses a boggy area. At the distal end of the loop, on the steps leading up to an observation platform, some mammal (?raccoon) had deposited scat. Butterflies love scat, because it provides them with a source of minerals they can't acquire from nectar.
This Baltimore Checkerspot was so totally absorbed in his feast that I was able to sit on the step next to him/her and place my camera lens about two inches away for some nice macro images (try supersizing them to see how beautiful these creatures are...)