This was a quick plugin I put together last night when I got too frustrated to work on my actual jobs. It lets you define a single shortcut key which, after you input your Instapaper username and password, will let you send the currently-selected article in Google Reader straight to Instapaper. It works fine with themes like Helvetireader (Safari extension), and functions in both list view and expanded view.
This is a companion discussion topic for the original entry at https://brettterpstra.com/2010/07/28/single-keystroke-instapaper-in-google-reader