Someone at work mentioned that Outlook was complaining about an RSS feed. The error message was "The link may not point to a valid RSS source". This was odd because the feed validated and every other feed reader was able to handle it. Even stranger, Outlook had no issues with the same feed if it was not coming from an https address.
After some experimentation, I tracked the problem down to caching. The feed was being generated by a PHP application that was using sessions. By default, PHP sets some http headers to prevent pages from being cached when sessions are used. Outlook 2007 could not handle an RSS feed coming from a server using SSL with caching requested off. The solution was to either turn off session handling when the request was for an RSS feed or change the headers before sent so that caching wasn't turned off. For this application doing the latter was easier.