Chances are someone already put this out there, but I didn't find it anywhere, and Amazon themselves are burying the lede in confusing copy. Top of the US front page right now tells you that you can get $20 store credit on a trade-in of other leading ereaders, but if you click through there's a button to apply a $20 discount on an order of any current Kindle.
http://www.amazon.com/gp/feature.htm...pf_rd_i=507846