The mesh had a few problems.
In edit mode, select all vertices then execute Remove Doubles and Recalculate Normals.
When using parent with Auto Weights, its best to have some more bones, the other leg bones and some body bones for example, or else the auto algorithm will just assign all vertices to the one bone.
In the file you posted, some vertices were weighted just a bit to the leg bone, because they were not weighted to any other bones the influence was 100%.
You can remove all the weight easy by using the tools in the Properties > Data > Vertex Groups panel, while in edit mode.
In weight paint mode, the brush blend was on Multiply, set to Mix for a more normal affect.
Heres a modified version of the file: http://www.pasteall.org/blend/31653