The Reading List is just a folder named "Reading List" within your Bookmarks folder; anything that can create a bookmark can add something to it. (It's actually all your bookmarks that are iCloud-synced, not just the reading list.) I imagine there are a bunch of apps with code for an "Add to Reading List" button already written but #define'd out until iOS5 is released, to avoid confusing people who press it and don't get any syncing.