I think this is a strong concept but could be greatly enhanced by the use of the mobile internet. SMS is a limited medium that is great at supplying small amounts of data, but is poor if you wish to interrogate data or list large amounts of data. In the case of the Amazon service if someone mispelt the text message or it brough up a large number of results the SMS product would fail. Whereas a WAP page could list the many products, inlcuding recommendations (which are so popular on Amazon) and deliver a richer user experience. Furthermore if you were already a registered member of Amazon there is no reason that you could not integrate the username/password and stored credit card details to make the order on the mobile.
I have just completed a similar deal with a major online retailer and would be pleased to talk about this to you, if interested.