I think some of the answers above serve to highlight the differences between those that have experience of road riding, particularly club cyclists who have been brought up through 'the system', and those of us who have merely bought a mountain bike and just kind of got on with it.
I haven't the faintest idea of 'road etiquette', but do occasionally meet those out on road bikes whilst I'm riding between trails or commuting. Even when I'm giving it some pan-handle on the mtb, I'd be very surprised if a roadie jumped on my back wheel for any length of time. I'm pretty sure that they'd realise quite quickly that, despite my reasonable speed at the time, I'd be unlikely to sustain it for more than a few miles and that I'm most probably using all the wrong gears at the wrong time or something, and they'd bugger off past me as I'd be no use to them anyway. Conversly, if I happen to catch up with a roadie then they're usually either daydreaming or spinning some life back into a pair of heavy legs. As soon as they twig it's not another roadie then they also bugger off aswell.
Of course, I always pass them later on as I crash the next red light or go past on the pavement as they're waiting in traffic… :wink: