Totally agree with Fred here...   
Postrolls are are really an untapped opportunity.  After you watch a
funny baseball clip, a link to buy tickets or sports jerseys makes
total sense.  Even better would be if you let users pick the link.

Feedburner lets me approve or bag the ads that appear in my feed and on my site, and I've turned down a few penny stock links here and there... and I think that improves the overall quality of the experience and content on the site.  YouTube should work the same way, letting me turn down advertisers I don't believe in.