I use the proxy-based ad blocker GlimmerBlocker and visit sites, including Macworld, that use Pheedo. I noticed that the Macworld vodcasts were turning up empty in iTunes. The cause, it turns out, is that GlimmerBlocker blocks Pheedo.
There is a workaround though -- just create your own list, and add a whitelist for host www.pheedo.com, path /click.phdo with priority 3. Then any Pheedo-using sites will work just fine.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2009020500155714