For instance, you could look into some APIs for the most popular submissions on Hacker News and reddit and present them as articles that need summaries.
You could also create a bookmarklet that users or authors can use to recommend an article to be summarized, which will then appear in some kind of summary queue on the main page. (Predefined) tags could also be included, so people who are only interested in summarizing certain articles, can cherry-pick these.
Another thing is to suggestion is to reward productive users and upvoted summaries with a link to the writer's flattr profile. People can do this, even though the person has a flattr profile now[1][2], and I think this might increase the incentive even more.
If this takes off, you could create some browser extensions similar to the Hacker News Sidebar[3], which displays a sidepanel on websites that have been submitted to Hacker News.
[1]: http://blog.flattr.net/2011/04/opening-the-floodgates
[2]: http://blog.flattr.net/2011/05/flattr-almost-anyone
[3]: https://chrome.google.com/webstore/detail/hhedbplnihmkekhgma....