My dog does that too. I wonder whether it's the collie in her--kind of a herding thing. If I catch her at the right moment and ask her to sit, I can stop her. Perhaps just asking for another behavior and rewarding that one might help? And have you tried ignoring her when she does bump you? If you make a big fuss--even in a negative way--you might be helping to re-inforce the behavior.

Also, you might find helpful advice in Patricia McConnell's _The Other End of the Leash_ which talks a lot about how to "read" dog body language.