I think it depends a lot on a) the person asking, and b) the context. I agree with what others have said that if the person is someone close to you, it isn't necessarily rude - I think it's a common thing to be curious about such things, although I think it would DEFINITELY be better if they didn't assume people WERE going to have kids, i.e. by asking "Have you guys started thinking about whether you'll have kids?"
I think the context matters, too. I think it's to be expected that right around the time of the wedding, a lot of people will ask as they get caught up in the mood of the festivities, and hey, if you've invited people to such a personal event as a ceremony in which you commit your life to another person, it's somewhat understandable that they'd feel more comfortable asking personal questions. But after that, I think it's rude to just ask outright, because really it just seems nosy and gossipy. If you're having a conversation with someone in which the news of someone else's pregnancy comes up, it might be appropriate to ask, "Hey, are you guys thinking of having kids right away?" But just out of the blue prying is kind of rude. And the repetitive, every-holiday-or-family-get-together question that many couples who are childless and married more than a year become accustomed to is just plain downright rude. For all they know, the couple could be trying to get pregnant and are having a hard time, or are undergoing emotionally wrought fertility treatments, or maybe has miscarried a time or two...asking "so when are you going to start having kids?" over and over assumes that the person asking already knows that the couple *hasn't* started trying, so unless you know for a fact that they have not, I think it's best to keep quiet.
I'd be interested to hear if others disagree with me, though!