Jason,
I won't claim to have any rock-solid answers, but I have been led to believe that with Stumbleupon as well as Digg, the "clout" of the person who first discovers the site has a lot to do with whether or not that post ends up going viral.
I think most of us can attest to this as we've all seen stories that weren't all that great, but they still made it HUGE on the social networking sites.
Specific to Stumbleupon, I believe you are right about their algorithm taking into account how many pages from any one domain have been stumbled. That is one of the reasons why I only submit vote request for posts that I really think will take off, or that I really want to get a lot of exposure for.
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