Nathan Weinberg, here's a shocker: Wikipedia isn't the only major site that uses "nofollow" on outgoing user-generated links; YouTube does it too. I looked at the source code on a typical YouTube video page and discovered 137 rel="nofollow" tags!
The strange thing is, YouTube doesn't reserve the nofollow for outgoing links, they even slap it on links to other parts of YouTube. For example, the Related videos box in the sidebar, as well as the links to user's profiles, all receive nofollows, even though those links present no search spam value.
If you do want to link from your YouTube video to your own website and keep the PageRank, fret not. Turns out, if you paste a link into the video description (the full link, including http://), YouTube will automatically turn it into a link, and won't slap on a nofollow tag. Take advantage of this loophole, and be sure to link to your blog post when posting a video.