Waffles describes fighting information rot as one of the great challenges for 2011.
The biggest problem with old and much-voted-on questions is that it's very hard for new answers to change the "status quo", i.e. gain recognition for new, more up to date, meaningful contributions. Even though the old question gets bumped, most users (I'm fairly sure) have the "order by votes" view active, and will never see the new contribution, or see it on the bottom - where they are mixed with those old answers that weren't upvoted because they weren't good.
To improve the situation, I suggest showing the two newest contributions on top, regardless of the sorting order the user chose, and visually marking them "new answers", for questions older than a certain number of weeks. Either in their full form, or in a shortened form as suggested by Marc Gravell:
This would have to be for a certain time only, say, until the most recent contribution becomes old itself, or has been viewed a certain number of times, or has been voted on to some extent.
This would allow for new answers to gain equal attention like the first answers when the question was still young, something that is sorely needed. At the moment, it is almost impossible to get a new, newly correct answer into a strong existing "status quo".
I realize this would be a major change for some, because it would mean taking away a part of their control to display contents as they damn well please. But if it helps making old questions more accessible to new contributions, I'd say it's worth it.