The guy doing it stole hundreds of Yahoo IDs by tricking people to visit his websites, where he read and copied their Yahoo cookies.
He has amassed an inventory of hundreds of posts, which the program posts to various forums.
For more evidence, pick a suspicious post on the Yahoo Finance Message boards for CSCO or SIRI. Select a unique-appearing phrase or word and do a search on it. You'll likely find that the same identical post has been repeated many times, under different IDs. The link above is an example search on the term "McDemocracy," which is used in 10 pages of identical posts.
It is rumored that the perp may have something to do with Noam Chomsky's website.
Leave it to a liberal to ruin something good.
This is not spam, it's a legitimate concern.