A little truth about Yahoo Stock message boards...
As someone who was a poster on Yahoo and the other stock chat boards, I can tell you that they could be a popular forum for discussions.
(Hint: Just ask Informix's former CEO's (Dexmier, Finnochio, and even Gynes) about Gumby. ;-)
The point is that if you can control the board, remove the "flamers", "spammers", and idiots who are trying to make an off topic political statement, then its possible for these boards to have a decent forum for discussion.
If you know the industry and the company, its very possible to make posts that could effect the price of the stock. However, its very difficult to do so.
As an officer of a competing company, posting to chat boards, or even maintaining a public blog, could be dangerous. In this case, IMHO, the SEC should look in to his attempt at manipulation of the stock. Especially if he posted during the time that the acquisition took place.
If it could be proven that he posted when he had knowledge of non-public material events, then he's definitely in hot water.