i'm an athiest and i'm very upfront about this.
though...
depending on the audience i'll sometimes back down to agnosticism as the most rational position...
but then it all depends on what you mean by 'god' and i'm most certainly an athiest on 'omnigod' so unless i'm discussing the issue with a sophisticated philosopher / theologian (which i find does indeed inspire a sense of the mystical in me) for all intents and purposes i'm an athiest.

now...
i won't claim that this wins me any friends...
but i am very upfront about my beliefs.
if other people are talking about their religious beliefs i might keep my trap shut... but i won't pretend like i'm agreeing even though i am in fact listening politely.
if they ask me what i think then i'll tell them.

the same issue (for me) comes up when people talk about astrology. horoscopes and belief in ghosts and esp and the like. if people are enthusiastically agreeing with each other then i'll keep my trap shut (mostly!). but if people ask me what i think then i'll tell 'em.