The generated RSS makes use of Microsoft's new Simple List Extensions.When used with an SLE-aware browser or news reader, these extensions allow the wishlist items to be easily numbered, sorted and filtered. The newest beta of IE7 includes complete support for SLE and these wishlists look great inside.
One very cool aspect of these wishlists is that they are built using ECS, and that you can use the same technique to build something very similar (or totally different). Take apart a wishlist URL, and you will see that it consists of a call to ECS, including a reference to an XSLT style sheet developed by Amazon. This style sheet takes the normal ECS output and generates the RSS, complete with the extra tags needed to support the Simple List Extensions.
Update: Watch Amazon's Drew Herdener discuss and demo this cool new feature.