That does seem to be true.
Fanfic writers have a theory for that... that Clark sort of softens his body or makes himself less dense when he knows someone's going to hit him so that the blow is absorbed a bit.
Though... there was one time in the series... not sure what episode... where Jimmy runs into Clark while Clark's not paying attention and it really hurts Jimmy's shoulder.
I'd like to think that it hurts Lois *after* she knows because Clark's being a brat.
Sara