If you only look at the surface, perhaps. But there's a lot under the engine when you think about them archiving your past searches, your past clicks, etc. in terms of aiding search-based memory. At minimum, it's just the convenience of re-searching on a term you searched on a month ago and instantly being able to see that site you went to for the product or information you wanted. But the possibilities go well beyond that into each user developing a search profile the more they use it ... and the more your profile develops, the smarter Amazon can be in giving you the search you want. This is competition for G's Personalized Search -- Amazon already knows a ton about me, now let them start using it to customize my searching.
And that doesn't even begin to go into the integration of search and commerce here. Isn't that what G wants to do, essentially, with Froogle? Here comes Amazon, and they take it to a new level -- they don't point you to web sites where the product can be bought a la Froogle, they sell it to you themselves! It's just books now, but surely that limitation won't be around too long.
Google mission: "Organize the world's information and make it universally accessible and useful."
Amazon mission: "Allow people to find and buy anything on the Internet."
I kinda think this A9.com is where those two mission statements start to converge. Question is -- how does Google like having its results used and enhanced like this? Is this that point where the partnership turns into a competition?