Forum posts are only reliable returns if the poster is knowledgeable and the forum itself is in the same industry - I'm betting Paul O'B and Rudy (r937) get more than their fair share of traffic through their signatures based solely on the quality of their posts and their knowledge of the subject matter. If you're looking for a return when you're just making forum posts to random sites, then the return's going to be negligible at best.
You might get some returns on "advertising" posts (if the forum allows them), but again, that's only going to work if the subject matter is of some interest to the community the forum supports. An advertisement post for a new baby food company will get a much better return on a "mommy blogger" centric forum than it would here. Likewise, a new PHP CMS product would get more return here (again if it was allowed here when it's not) than it would to a "mommy blogger" centric forum.
It all depends on the audience. If it's relevant, it'll have a chance to work. Otherwise, it's just noise.