Great article from the PSYBLOG.
Despite their bad name, stereotypes can be handy short cuts that give us useful information about the world and other people. For example the stereotype of psychologists is that they are going to analyse you, then start meddling. There's certainly some truth to that, after all that is their job.
What this study demonstrates very neatly is just how sensitive we are to the minutiae of social interactions. Subtle cues from the way other people behave and more generally from the environment can cue automatic unconscious changes in our behaviour. And by the same token signals we send out to others can automatically activate stereotypes in their minds which are then acted out. As much as we might prefer otherwise, sometimes stereotypes can easily influence our behaviour and our conscious mind seems to have no say.