Like many before me, I've noticed that some questions, which start off bad and take a while to get fixed. Gather a ton of down votes before their state improves and then take ages to get to a positive score. (That is, if they ever do)
A lot has been said on this topic already. The interest has been rather large (a total of 88 upvotes on those 4 questions combined), but so far these questions have gathered a staggering total of exactly one answer with no less than two up votes. This answer seems to boil down to: it's too difficult and people should use their downvotes more carefully.
Be that as it may, I would like to offer my own suggestion to solve the problem. Additionally to indicating revisions in vote history in a very non intrusive way as Ben Brocka suggested, I'd like to see a notification that's a little more obvious when someone reversed their down vote on a question you've down voted too. This way, People who are a litte more involved can look at every reviewed question they've down voted and people who are less involved (or just happen to have a little less time at the moment) can still help the question get back on its feet more quickly when it improves its quality.
I see no reason why questions why are perfectly fine should spend time having a negative score just because they were originally bad questions. (I did assume that SE cares more about questions than it does about users) Nor do i see a reason why people who were perfectly righht down voting a question should spend their precious time manually hunting down questions that might have improved since they last saw them.
I'm open to alternative suggestions to solve this problem as well.