Matt Yglesias uses the turnover in the office of Senate President pro tempore to complain about that the current presidential succession plan is both foolish (#3 after the president is someone in a ceremonial role that you qualify for by being superannuated?), and perhaps unconstitutional. Fortunately, there's a plan already drafted. The Continuity of Government Commission -- a bunch of big names from past Congresses and administrations, with political scientists Tom Mann and Norm Ornstein as the major players -- released their report on the presidency one year ago. While their main focus was, as the title of the commission suggests, focused on the dangers of a catastrophic terrorist attack.
The Mann/Ornstein report suggests removing Congress from the line of succession. Furthermore, they also want to remove the tail end of the cabinet, limiting cabinet-level succession to State, Defense, Justice, and Treasury (in that order). After that, the commission suggests that presidents select in advance "four or five" additional people to complete the line of succession. This would have to be an actual office (presumably unpaid!), nominated by the president and confirmed by the Senate. It could be filled by prominent party politicians -- governors, for example -- or it could be filled, as Mann and Ornstein envision, by senior party politicians. So Barack Obama might designate, oh, Bill Clinton, Al Gore, Walter Mondale, and Tom Foley. This would serve the commissions goal of making it harder for a terrorist attack to disrupt the American government (by spreading the line of succession around the nation, instead of concentrating them in Washington). Beyond that, it certainly seems likely that if the president, vice-president, and the Big Four cabinet officials were all dead or incapacitated, Walter Mondale would be a better choice for an emergency president than, say, Tom Vilsack.
(Yes, Laura Roslin was a terrific character, and perhaps a terrific president, but still...).
The commission also made suggestions for other weak areas and potential loopholes in the current presidential succession scheme, which is based on a 1947 law (along with the 25th Amendment). Before 1947, by the way, Congress was not part of the order of presidential succession.
I think the commission's suggestions are a solid solution to a real weakness, and Congress should enact them. Unfortunately, as far as I can tell from the commission's web page, little if anything is happening...but it should! Whatever one thinks of the late Senator Byrd, I can't imagine that anyone thinks it's been a good idea for him to be close the presidency over the last couple of years, and the same was true of Strom Thurmand when he was president pro tem. Hey, Congress! It's been almost a decade since the September 11 attacks -- fix this stuff!