I have mentioned this [or meant to] a couple of times. Glad to see that others have been verbalizing this as well --Techdirt:How Amazon Opens Up And Cleans Up
And let's not forget Google Web services. My guess is that the "killer app" is not going to come from a web service API Amazon puts out, but some kind of an app that uses a few webservices together, in a way that none of the companies providing them could by itself. Like something along the lines of "an applicatin that provides book reviews from wiki, amazon, etc depending on your preferences on amazon, your blog links, and what top news are on google combined with you and your circle have been searching for, and whether it is cheaper to rent from Netflix and Wallmart or buy new and resell on eBay later." or something like that.