I'm glad I read this thread, because if there was a cake sitting on my table the whole time at the reception, it would not have occurred to me that I wasn't supposed to eat it whenever I was ready to. I guess I wouldn't have connected "cake sitting on my table" with "wedding cake that the bride and groom cut." To me they would have been two totally separate things--yes, at all the weddings I've attended, the bride and groom cutting their cake is a big thing, but I would never have realized they had dominion over all cakes at the reception. I'm afraid the only thing that would have kept me from tackling that cake on my table was not wanting to look greedy to the other guests by being the first to suggest cutting the cake!
For brides planning to use this idea in the future (and I think it sounds like a lot of fun, especially if you're going to invite me!) I think a couple of cards at the table asking people to please not eat "the centerpiece" until later would be a good idea, just in case it doesn't occur to anyone at the table to wait.