This is the one that seems to be exciting me the most these days, when I look at my refs and I see that direct type in I assume we must be doing something right. We don't market the site in any other way other than our organic SEO efforts so each type in is almost always a repeat visit.
Although direct type in traffic can be driven by a variety of marketing initiatives its not common practice for spammers to invest in mainstream media to push their MFA's. My thinking with website development is leaning more and more towards getting the visitor to visit us again and less about a speedy transition to an ad click or sale. Keeping our users engaged has become a big part of our focus, I'm not just thinking content I'm also thinking functionality, what unique slant can we offer, research tools and the like. If the users keep pushing your URL I think your on the right path in the long term.