The latest version (1.5) of the iPhone app Feeds (iTunes Link) now has Read It Later integration.
Feeds is a Google Reader RSS Reader for your iPhone. After saving your Read It Later account in Feeds’ settings, you can save pages directly from Feeds into Read It Later. The settings page can be access through the main iPhone settings page.
I believe this is the first app to integrate the Read It Later API, but several others are coming very soon. If you’d like to see it in your favorite app, let the developers know there is an API available.