you won't be treading the line of danger with the people who hate "thank-you nodes".
For all that, I tend to agree that a responding node is more appropriate. For one thing, there are frequently several solutions that work; for another, threads often have a fair amount of cross-talk between answers, which will tell you if something's either incredibly brilliant or mind-bogglingly stupid (somebody will presumably have replied to that effect).
And I'm somewhat leery of marking a node as The Answer--if somebody else comes by with the exact same question, they benefit slightly, but somebody with a subtly different problem may read the thread, jump straight to "The Answer", and miss the node down below which warns them about the limitations of this approach (disaster ensues). Granted, that's very hypothetical, but I don't know that it's that far off.
So on the balance, I think this idea sounds good but would have at best a small positive impact--besides, do we want another kind of voting for people to get annoyed about?
If God had meant us to fly, he would *never* have given us the railroads.
My thought was to have the node color coded or gold starred or moved to the top, so that it would stand out for people searching nodes. And prevents the clutter of a thank you message... which some node get very cluttered... most nodes aren't that bad... but something like this would really help in places like Dereference an array reference