Questions/Pekka made a good point in a comment reply to Jeff'sJeff's answer.
You're talking about users going "beyond the call of duty", seeing an answer through to the end and making sure the OP is satisfied with the result rather than just posting an answer and leaving it at that. Questions made another point in the same answer, that these types of users are aware there's no reward for these actions yet they do it anyway so they're probably not looking for anything in return other than their own sense of self-satisfaction.
That being said, don't we tip professionals that go that extra mile to provide the best service possible? For that reason, I've given your question a +1, but really I'm only just tipped that way and I don't see anything like this being implemented.
I would offer one suggestion of silently showing your gratitude - visit the user's other posts and see if they're deserving of any upvotes too. Just make sure you're not doing it in a stalker-like manner ;-)