Sorry, SoCalVal, I didn't mean to insult you.
I'm still trying to understand, though, why you cared when people cut and ate the cakes at their tables? I know I am missing something. Really, I am sure you did have a good reason. And it worked out beautifully, and your guests had a great time, and that is what is important.
If you (general "you") put food on their table, I think you have to expect that people are going to eat it when they want to, without knowing they are supposed to wait for some kind of signal, same for the dessert as for a basket of rolls or a dish of nuts or candies, all of which I often see on tables at wedding receptions. (Not if it were The Wedding Cake, of course.) At a Jewish wedding, usually someone (often Grandpa) makes the blessing over bread (=the meal), and there are always a few people who don't know or forget or don't care and get into the bread baskets or salads on their tables before that, and the sky doesn't fall. But really the only way to avoid it would be not to put anything out until you want people to eat it. That would ruin your terrific idea of using the cakes as centerpieces, though (that is SO clever).
I've been to many luncheons where the desserts are already on the table when you sit down, for logistical reasons, I guess. You just eat it when you're ready for it. If I'd been at your wedding, I wouldn't have guessed you intended anything different from what I was used to seeing.
I'm not sure what would be the best way to ask people to wait. Maybe a little card on the table saying something like, "Please cut this cake along with us when we cut our wedding cake" or something like that (I'm trying to avoid something phrased in the less-hospitable "wait" or "do not" kind of wording). I do feel that mentioning it on the website isn't enough. Some people might have looked at the site before such details were posted. And even those who look later may miss it; wanting "to know more about the happy couple" does not always translate to reading every word of every page of a (frequently lengthy) wedding website. I'm sure you didn't mean to say that guests who don't read them thoroughly don't care about the brides and grooms as much as you do.