I've just released a tool that some Calibre users may find useful.
Erudite will download articles from Instapaper, convert them to an appropriate format such as EPUB or MOBI, and add them to your Calibre ebook library. It is a command-line driven tool.
Home Page •
Downloads
(While this is not an actual plug-in, I thought users may find it useful, and I couldn't find a better sub-forum to post this under.)
E.