social etiquette query

I know that it's rude to tell someone when they're being rude, and, per Miss Manners, I will usually try to find a subtle and polite way of indicating when someone's behavior is out of bounds in a social situation.

Sometimes, the person is someone I feel like I can talk to about it after the fact -- most obviously, a partner falls into this category, where I can go back and talk about it later, especially if it's something that could come up again and I'd rather it didn't.

But what about when it's a friend? Do you bring it up when you see a friend behaving rudely or in a way that bothers you? How do decide to bring it up, and how do bring it up once you've decided?

Would you want someone to bring up some social faux pas to you? If so, how, and in what circumstances?