Oddly, I haven't seen it until now. Maybe it was posted in a different forum originally? I watch the Website Comments forum most closely.
I think the idea is good, but the biggest issue I see with it, as someone already noted, it would become a popularity contest, as well as people putting focus on getting more votes to obtain the special rank. Even if there's no added features or benefits, the mere fact that there'd be come visible "rank" would be the benefit people would strive for... status.
Now, I can think of better ways for this to work, but it isn't something that is easily implemented without having major modifications (MODs) to the forum software. And some people already know how I feel about modifying the software (maintenance mess/nightmare). But it would be nice if an original poster had the option of indicating which 1-3 posts were the most helpful. This would be similar to sites like Yahoo! Answers or Ask.com, where the person asking the question marks which one is the "best answer". BUT, that's not something I see being implementable with the features phpBB has built-in.
-Alex