Change in RSS 2.0
Dave Winer: This morning an esoteric update to the RSS feed generator in Radio. We now omit the xmlns attribute on the rss element because some parsers, especially homegrown parsers, can't correctly interpret it. For those interested in more detail on this, I explain why here. Now, of course, all such homegrown parsers which upgraded to support the new RSS 2.0 namespace are broken. As Shelley said, quick hacks are not the solution.
Dave continues to say, I believe Sam's feed is not generated with Radio. http://radio.weblogs.com/0101679/rss.xml clearly states "RSS generated by Radio UserLand v8.0.8". Those that care to can see the code here.