It sounds like nothing more than an employee mistake that's getting blown out of proportion. A customer asked Party Pieces, the party supply company owned by Kate Middleton's parents, if it would "be selling traditional British-themed products" in the coming months on its Facebook page. The company's reply was "We certainly will be doing some traditional street party bits. These will be available mid-late February."
The story passed relatively unnoticed in Wednesday's Daily Mail, but Vanity Fair summed it up and added a little flair today, saying Party Pieces "is reportedly preparing to release a line of royal-wedding-themed products."
As far as I can tell "British-themed products" are British flags, miniature castles, and Keep Calm and Carry On posters. I suppose it is tacky to sell paraphernalia for their own daughter's wedding, but it's probably something the company does for other royal celebrations. Either way, it's a far cry from starting a line of commemorative plates!
I'm really not defending the Middletons; it's just that I don't believe the Queen, Buckingham Palace, or their very own daughter would allow it. The family has denied all allegations, and I just don't think there's enough incentive to shake the crown. I mean, they'll probably be made baron and baronness or something, and that sounds much better than a quick profit.