Great quote from Aaron Wall of SEOBook.com about how websites who are busy playing the Digg / Reddit game are likely to lose out over the long term to people who are working to establish brands and credibility:
"Most people coming to your site from linkbait have a fly-like memory. One visit, one pageview, and they are gone forever … Google is looking at how natural a site’s link growth profile looks like, and discounting many of the rapid growth spikes if they are not followed up by an increased baseline link growth rate … Compare linkbait to developing a real brand. Developing a real brand is slower and more expensive, but search is intrinsically tied to branding. If your brand is the keyword, it is hard for search engineers to take it away from you. They are irrelevant if they do not show you at the top of the results … as you build brand awareness, it causes a smooth natural link growth profile, which helps you rank better for the generic phrases."
There are still plenty of reasons why getting a traffic boost from Digg or Reddit can be useful, but such short-term gains will increasingly pale in comparison to the long-term value that you can build by providing original news and tutorials, creating useful tools and engaging with your visitors time and again.