I haven't kept up with this story, but when mhking posted the article it triggered my memory of this case. I think it was the first case ever where a persons written thoughts convicted him.
Here is an update:
July 17, 2003 update: A state appeals court threw out Dalton's guilty plea -- not on First Amendment grounds, but because the court ruled that he'd had ineffective counsel at the time of his plea. He still might be re-tried.
Thanks, flutters. The whole thing smelled pretty stinky to me, including the part about forfeiting his right to appeal.