I'm a newbie and don't know how incentives for add ons work (is that what your question is about?). But, I was on the Magic Dec 6th (7 night), and went to one of the presentations. Here are some notes I took for purchases:
160 points minimum for BLT
Points would be deposited in Feb, 1st payment due March (with their financing, I assume)
$5/point credit (fine print says it comes in form of gift card that is to be used toward down payment)
No Dues until Sept
Matching points (assume developer points?) must be used by Oct 3 09 (or cruise within 2 years)
$500 ship board credit
Beach Towel
They told me they were also selling AKV, but didn't talk about it during the presentation... they were really talking up BLT. There was an $8 incentive for AKV. Prices were $112 BLT and $104 AKV before the incentive credit, and told me AKV was going up to $112 in Jan.
They told me that if I decided to buy in the last day or two (or did a "mail out"), that they would apply the shipboard credit toward the purchase price (or down payment?) since it would be basically too late to enjoy the full $500 onboard credit (unless I'd already run up my bill or used it on a last minute shopping spree, I guess

). Apparently, though, when I asked a few more pointed questions, they clarified that the $500 OBC was only good for contracts over 200 points.
Anyway, I'm glad I did not purchase on board... I am getting so much more for my money going with a resale (and I am able to get a small contract).