A very frustrating problem! We've had an RSS feed available at www.lowcarboneconomy.com/GetRSS.xml for ages to syndicate our news feed. I monitor it daily using Netvibes to ensure that it's working. The other day, I created a LinkedIn and FriendFeed page for LowCarbonEconomy.com and neither would accept the RSS feed.
So, W3 validator to the rescue again. Or not. I got a 'misconfigured server' error. I couldn't find any info on this via Google, so tried a different validator. This time, I got a couple of warnings, but a valid feed result 🙂
So, to cut a long story short, I narrowed it down to some kind of HTTP Header or compression problem and asked Andre to check it out. I'll leave him to pick up the story: http://drayblog.gotdns.com/...
(SPOILER: In case his blog goes offline, it's all about IIS compression and setting a Content-Length in the header to avoid chunked compression by turning the content from dynamic into static)