It seems to be a different kind of upvoting. When I write something new it generally shows up on both reddit and del.icio.us/popular. Stuff about Ron Paul only shows up on reddit. What that implies is that people are upvoting it because they want other people to read it, rather than because they found it interesting themselves.

The reddit guys actually worried about this problem from the very beginning. That was why initially there was such an emphasis on voting as a way of training filters: if you voted for dumb stuff, you'd be punished by having to read dumb stuff. But the filter-training message never got through, perhaps because the filtering worked so badly for so long.