Wow. My first ever "pull over" today from a woman who was delayed possibly by 30 seconds - and that's in quite a lot of cycling. Country road rage.
While I will pull over if there's a build-up of traffic and no way for them to overtake without me doing so, I was a little p*ssed-off and considered a gesture in kind. I'm tempted to just ignore them though the automatic response to give them the finger has to be kept under control.
* Flip 'em a finger
* shout back (too late by the time they've been heard)
* or ????