I think it depends on the two religions involved and how strict both parties are. Some are less compatible then others.
As long as both are willing to give a little and not try to convert the other person at every chance they get I think no big deal. I do think though that talking about how you will raise kids ahead of time is important. There might be a lot of family pressure at that point.
As long as you keep in generic and non-judgmental the subject should be just fine.
ETA: I guess the question got deleted while I was posting, too bad. It could have stayed out of the forbidden zone if everyone could be an adult about it. Of course that could be asking too much.