I catch myself doing this sometimes -- it's not so much becoming a different person so much as it's emphasizing different aspects of one's self to fit into different contexts. I always feel guilty when I catch myself doing this, but the truth is that everyone does it to some extent.

An extreme example is the way you use a different tone when you're talking to your boss vs. a close friend -- it's the same kind of discrepancy in self-expression, except it doesn't necessarily stem from the external context of the interaction as much as an awareness of the specific individual(s) you're talking to...if that makes any sense.