I've been trying to use reading list as a replacement for Instapaper and ReadItLater (just to see if it can be done). The main thing that is missing is the integration with every major app. As far as I can tell there is no public API for the Reading List, but there is applescript support. You can add articles to the reading list like this:

tell application "Safari" to add reading list item "http://jga.me"

which gives you:

If Apple exposes an api, someday, I'd probably use ifttt to sync up ReadItLater and my reading list. Until then I'm trying to think of ways to sync up the two. Maybe just a script that I run on my mac, hits the ReadItLater or Instapaper API and then runs an applescript script for each new item. I'm still kicking around ideas, has anybody else come up with a good solution? I might just have to go back and use one of the 3rd party services, which isn't necessarily a bad thing.