Both gaining an advantage is irrelevant. It is malicious when it's unwanted, not communicated, and not accepted by both drivers. If I came across a report with bump drafting I would penalize the person who bumped, if the person that was bumped submitted the report. This means though that I would have to actively do reports again, and that probably won't happen much until after July 2010....when my degree finishes...
I think if people don't want it then they should tell the person after the race, then if they did it again they should report them for it. Personally I don't like bump drafting unless it's on the oval and not in a single seater.