This is a great example of how instant communication tools like Twitter are being used for commercial means. Basically the Amazon and Woot deals are feeds of offers, pushed out via Twitter. These kind of feeds could also be received in your RSS Reader, or email.

The next step for Amazon and Woot is to allow personalized Twitter feeds - e.g. you could sign up to an Amazon Twitter feed of just music deals (if music is what you want to buy). This would equate the "followers" of Amazon's Twitter feed to leads for the company - very fine grained ones, because Amazon knows what their "followers" want, based on which e-commerce deals they sign up for via Twitter.

I must mention that in my advisory work with nooked, an Irish RSS marketing company, we're working on a concept just like this - which Nooked is calling FeedCommerce. The concept is basically that personalized feeds allow consumers to get great deals and retailers to specifically target their goods and services to consumers. Twitter is in a great position to take advantage of this trend, as an output pipe for such feeds. It's yet another example of how a brand new communications tool like Twitter is being used in ways that few people would've predicted, least of all Twitter's developers.