Thanks to all of you for your comments. I won’t have time to respond to everyone personally, so I’ll respond to people in general.
First of all, my fiancée reminded me with some asperity, after I told her to check out this thread to see what people thought, that she never specifically said it was a good idea to put the registry information in with the invitation. She just said “do you think we should do that? Why don’t you ask your friends at the SDMB?” Also some of the invitees live out-of-state, and some live out of the USA (in Europe.) In any event, we are going with the general consensus, no registry information with the invitation. By the way, just last week I received a wedding invitation that had the registry information included on a small card, which is what prompted my fiancée’s question.
As far as the registry idea itself, I personally am in favour of it. When I go to a wedding, I like to bring a gift to the couple, and I feel better if I know I can bring something they actually want. When my brother and sister-in-law got married, they didn’t register anywhere, and they received around six or seven sets of drinking glasses, of course all different styles. What do you do with 70 drinking glasses?
As far as the “no guest” hint: everyone that is coming to our wedding is a close friend. We want to have a small wedding, and in the “wedding workshop” book we have, it says “pick a number of guests, and stick to it! Don’t keep on throwing in more people.” We chose the number 60. All the guests are people we know well. In the case of the people who don’t have a significant other, I checked on the list and these are all people that we see at least once a week (people from the dance studio where we take lessons.) So a new boyfriend/girlfriend is unlikely to pop out of the woodwork without us knowing about it. So the question is, do we invite only half of those friends, because we give them the option of bringing a date, or do we invite all of them and tell them they have to come by themselves? That was the basis of the question. Right now, our decision is to invite all of them, and just bite the bullet if some (or all) decide to bring a date to the wedding. Many people see it as the “social kiss of death” to show up at an event unaccompanied, and will make an effort to find someone to bring with them. The RSVP card has a line saying “number of people attending.” Of course, if someone puts in the number “10” and brings 10 drunken louts with them, we will get the other wedding guests to tar and feather them. 
As far as children are concerned, they are invited to the wedding. We plan on having a couple of “children’s tables” with drawing books and games. Teen-agers will just have to suffer with us adults. 
Oh, and by the way, zuma, maybe I’m cheap and rude myself, in which case my fiancée and I would make a perfect match. Never assume!