Digg adds nofollow to some links, Quote "We’ve added rel=â€nofollow†to any external link that we’re not sure we can vouch for. This includes all external links from comments, user profiles and story pages below a certain threshold of popularity." Take a look http://blog.digg.com/?p=864
For SEO you require quality backlinks i.e., links pointing to your from high PR website. DIGG is a high PR website ( PR 8) and "no follow" link mean it won't be passing the PR to you. So, this will effect your SEO.