can you submit a site that is not a blog on stumble upon? didnt find the option to submit 'favortie' site.....unless you have to upload a pic of yourself. etc..
Actually, its completely organic and legit traffic you get from social bookmarking sites. I did have quarried it from adsense team and they replied positively.
I agree the quality isnt the best, but its free traffic so I still think its worth doing, considering it only takes a couple of seconds to digg/stumble your pages.
You can use html-links in your profile - so that way you can get a link (no followed though) to your site. And if you just install the toolbar, you can submit your site.