If Amazon adds ability to pay with Paypal, I'll start buying from them tomorrow. Paypal is my main means of payment online, both outgoing and incoming, and Amazon has lacked much of my business because it didn't allow me the option. Once they open up to PayPal, I'm sure many people I know will also start to shop more Amazon immediately.
I like them too, mainly because it's me pushing the money to a vendor rather than them debiting my card. I'm much more comfortable doing that with some small company I haven't heard of before.