Tip o' the martini glass to the Bad Astronomer who links to a great piece of writing by Allyson Beatrice at Cocktail Party Physics called "Skeptic Etiquette." It's an amusing and charming look at how a skeptic or non-believer can deal with their credulous friends and colleagues in a polite setting. After all, you might want to be a fun, friendly voice of reason, not that cranky old nay-sayer.
On a related note, Rebecca at Skepchick posts a link to Tim Minchin's "Storm" now on YouTube. It's a fun example of what you really might want to say at that dinner party with the credulous person. Warning: contains some curses, so don't play it with other people's kiddies around!