E.H.
Actually, I tend to think the opposite. I think it is kind of "rude" to open gifts at a party because I think it puts an emphasis on gifts as opposed to the actual celebration. Plus, it has the potential to make someone feel bad if they can't afford to bring one; that may happen more in this economy. I think it is challenging for the younger one's to sit through it. And, for the older ones, I think it takes them away from the fun they are having.
I think more people started doing it so they didn't have to write thank you notes.
I think it is more important to teach the "birthday child" etiquette by greeting his/her guests personally, assuming they are over 4, and when each child is leaving, to say "thank you for coming to my party." I think it is the perfect time to teach children that they write thank you notes before they play with any toy. Maybe I am too old-fashioned. I think it is the perfect time to teach the guest to greet the birthday child with a "Happy Birthday" and to thank the parents for hosting the party.