Yeah, I know what you mean about the lip.
Check this:

Overview:

How to measure offset:


So the lip does make the wheel wider, but the main determinate of offset is the relation of the hub mounting surface of the wheel compared to the center of the wheel width. So yes, the lip makes a difference, but only because it makes the wheel wider. See how in that last diagram the spokes go inward, creating the lip. So spoke angulation can create or obliterate a lip, depending on the angle.

For more info go here:
http://www.tirerack.com/wheels/tech/offset.htm

Hope that helps a little.