I think you're right, Digg's front page probably sends a lot of link equity and gives a temporary boost in rankings for sites listed (until they drop off the front page).  I suppose you could also make the connection that the amount of "link juice" a site is able to send out can be based on, in part, user activity wheather GA tracks it or click throughs from the search results signal it or something else... once we do more testing we may have a better idea on what creates the apparent correlation.

In any case, it's good to be listed on Digg for organic puporses :)