You know I really fail to realize why Amazon's supposed entry into the digital music sphere will be a big threat to iTunes.
First off, Amazon has absolutely no track record of creating hardware. Even if they partner with someone, that someone still has to create a usable product, which few of the competitors has really been able to do. When Apple launched the iPod they had only a thin link with music, but they did have plenty of experience creating hardware, interfaces and software.
Second, the argument that Amazon has a lot of traffic will help sell their product seems pretty thin. Yahoo has more traffic than Amazon and their music service has really failed to gain traffic. Last I checked MSN was pretty heavily visited too and their current music site isn't so popular.
People talk about the strength of Amazon's recommendation system and how that will bolster any offering that they want. That's neat and all but frankly, I think that is inconsistent with the way that people actually listen to music. It is in my opinion the number one mistake of the subscription services in that there is this vast audience of people begging to hear new music. Most people dont perpetually want new music. They have their songs/albums they want. They might buy an album or two a year, but that is about it. There is not an army of people begging for new music. It is a niche audience at best and that is why the a la carte model of iTunes will be quite successful.
Also, out of all of the new 'products' that Amazon has launched recently such as a9, search inside the books, tagging, etc. none has really gained a lot of traction and none of them are especially fresh. Amazon is not really that much of an innovator these days. They succeed on inertia and a relatively decent price advantage. They would have none of this in the digital music sphere