Unsubscribing from the feed and then resubscribing to the link at the bottom of MO (which at least in Chrome on a Mac brings me to a gibberish page of code, but Google Reader knew what to do) fixed the problem. Maybe I was subscribed to a feed from way back when we were still called Mathematics.StackExchange? In that case, perhaps the explanation is related to the discussion about Mathematics.stackexchange.com redirect.
]]>