From their site for affiliates:

"With the launch of the new Amazon.com Web Services program, you can now enable your Web site visitors to add products to Amazon shopping carts, wedding registries and wishlists directly from your site [with an HTML POST form]"

This is good and all, but the business logic sucks:

"Items that are purchased through shopping cart buttons earn a 5% commission." This is yet another way for them to get around the "books sales directly driven by affiliates command a 15% commission."

They should just shelve the tiered commission structure at this time. No matter how hard you try, there are just so few occasions where you get to land the full 15% margin that it's laughable.

09/04/02 update: Jason sends me his data, I compare it to mine, and though our sites are quite different (his is a blog, mine is a site dedicated to SF&F books), we come to exactly the same ratio between 15% and 5% sales: 8.6%!