I had this problem for a couple of times now that in some area's not painted by weights hair is still showing up in the render (http://blenderartists.org/forum/showthread.php?t=205897) I'm still quite new with this kind of things so instead of just trying to work around it I'm kinda curious this is a bug or just malpractice from my side.

To exclude the fact that I may accidentally assigned some weight I did the following thing

- delete my current vertex group
- selected a small band around the neck and assigned a weight of 1.0
- inverse select and assigned a weight 0.0

But it is still the same output and hair appears on faces with a weight of 0.0 Changing the children type to particles fixes the problem. In the meantime I've tried this in blender 2.49b and their it seems to work fine.

OS: Windows 7 64 bit
Videocard : Radeon 5770 1Gb
Blender version : 2.56 official build from blender.org (but I have seen this kind of output also in older 2.55 revisions from graphicall)