I've seen that on social networks and usually find them to be lame and fairly annoying (personal opinion). I think they would be even less meaningful in a forum context.
Something more meaningful would be an indicator pointing out that a solution has been reached on a thread and possibly highlighting the post in the thread that best describes said solution. This would potentially keep users from reposting existing solutions because they overlooked the solution already given. It would also allow people visiting the thread (because they had a similar problem) more quickly find the solution information.
Highlighting the solution post would likely have to rely on the thread creator or an administrator and some threads don't necessarily have a "solution" as they are discussion related, but that's OK too.