Update: Got the following from my rep:
“Free item will not appear in your cart or during checkout. It will be on your order confirmation email and in your order”
So I get this emal from my Shop@Home affiliate rep, saying that right now, from August 1st through September 30, 2011, if you purchase 10217 Diagon Alley, you'll get a free MINI Hogwarts Express set, pictured above. There's no coupon code to activate the free MINI set, and it's not automagically added to the cart, unlike the Alien Conquest Jetpack set. I sent an email back but no word yet on how to get the freebie. Not sure if the MINI set and VIP points is worth tacking on tax and shipping to the cost of the set when ordering from Shop@Home, when Amazon has the same set for the same price with no tax to most states and free shipping, but people have done crazier things for exclusives.
Anyways, once I get word back on how to obtain MINI Hogwarts Express, I'll update this post.