Among most people I know, and among my own immediate family, talk about politics is constant, commonplace, and encouraged, even between people with different political views. Talk about sex is so common that I sometimes wonder if people my age talk about anything else.I was always told that it was rude to talk about religion or politics. (In my family, it hardly needed to be said that sex was a conversational no-no -- which brings up a question I'll pose to Nick: Who wants in-laws who are eager to talk about sex?) But there was this corollary: it's only rude when you don't know where the other person stands, or when you know they disagree with you. If neither applies, fire away!
And I'll admit that I think this custom, with the corollary, has its merits. As a non-liberal living in the liberal-Democratic world of elite academia, I regularly endure conversations with people who assume I'm as left-leaning as everyone in the room, and I'm forced either to smile politely at the Bush jokes or to show my colors and make everybody else feel awkward for assuming what they shouldn't have assumed in the first place. The rudeness isn't in the desire to talk about politics, or even (usually) in the substance of the remarks; it's in the narrow-minded assumption that if someone's at the same dinner party as you, they share your politics. It'd be nice, in other words, if the person I was introduced to ten minutes ago conducted a little background inquiry before turning to me, sighing tragically, and saying "Can you believe they've taken back the Senate?"
As for religion, I think it's hard for religious and secular people to talk meaningfully about it because they are approaching it from such different perspectives. You're either a person of faith or you aren't, and the whole point of faith is that it's not something you arrive at rationally. So a civil conversation about religion, to have any depth at all, almost has to take place on one side or the other of the religious/secular divide.
If you're talking across the chasm, the best-case scenario is for the secular people to be politely curious while the religious people walk the line between being defensive and sounding like they're proselytizing. And that conversation, as Nick notes, tends not to be very interesting or enjoyable for anybody. At least discussions about politics, even across the aisle, can start with the common ground of stipulated facts or universally recognized dilemmas.
Incidentally, it's been my experience that when everybody is on the same page religion-wise, conversations can be quite energetic. I've sat silently at more than my share of family dinners with my battalion of Catholic aunts bellowing commiseratively at each other about the awfulness of this or that priest. (Some of the Malcolm matriarchs yell even when they agree. Especially when they agree.)
Finally, on a related note to Nick's wife's theory that "white people like to talk about nothing," a friend of the Cabinet says that WASPs have dogs just so they'll always have something to talk to each other about. ("Where's the dog?" "Oooh, look at the dog!")