It never ceases to amaze me either - once they see you in their mirrors they close up to the car in front and go as far into the centre of the road as they can to try and stop an overtake. Stupid since nothings going to stop a bike overtaking if the rider wants to.

I do however like to give a thank you wave to those drivers that make a big effort to do the opposite and pull right into the kerb so you can pass easily. Always give these guys a wave.