When an answer is selected by the question-asker, I think it would be useful for people who are searching through to find answers to filter the answers by positive and negative.
For example, if Person X is wondering whether Activity Y is possible then they could search for affirmatively answered questions relating to that topic. With this system revisions could be made to older questions to if the status of said activity changes. An example I would to to draw your attention to is here: google map and siri implemtation , despite my answer being negative at the moment (i.e. saying that it is not currently possible) that could change in the future. Under my system, my answer would be marked as the answer, but would be as a negative answer, which would be open to revision at a later date.
Now I know what you're thinking: "We've already got the down vote", well yes, but the down vote is to show that you think that the answer is wrong, however my answer in the aforementioned example is correct at the time of writing, so a down vote would not be the right thing to do.
Thanks for reading.