I didn't read thread and I realize it is old but Digg and the other social bookmarking sites are all rigged. If you spend 20+ hours per week doing nothing but that you can drive some traffic, but no matter how interesting your submission if you don't have the right friends list and a huge IM buddy list of Diggers you have no chance of getting a lot of Digg's. All of the stories that hit the front page and/or get tons of Diggs are submitted by 'power digger' who submit tons of stories everyday and they are all friends with each other and Digg each others stories. The best you can hope for is one of these dudes see's your story and likes it and steals it and re-submit's it himself (which is how they get many of their submissions).
One thing these sites are great for is link building and getting sites or stories/articles indexed basically instantly. Google is constantly crawling every Digg submission, no matter how many Digg's it gets. Often times my Digg submission will initially rank higher than the article I submitted - the article usually overtakes it but it is great for a quick SEO pop, especially for breaking news and something that isn't competitive now but will be soon.
You can quickly add a link to a specific article on 10-20+ of these sites and if you are targeting something new - it will be very difficult for even huge sites to out-rank you because all Google see's is site A with 20 incoming links on a hot new topic and site B a huge, credible site but with only a couple of incoming links on that topic - Google will rank you higher.
I'm still
#1 after a month for a competitive poker term that just emerged, on my personal blog, doing this. My blog is Alexa 2,000,000 and the sites I'm beating are Alexa 3,000-30,000 and rank 1st for everything they want to in poker. But they just don't have the link juice I do on this topic - due 100% to my twenty minute social bookmarking campaign.