Well...... once again..... I DO NOT know laws....... I'm just usin' common sense...... I think.. hehe
Anywayz...... If he tries hard enuf, and has a decent lawyer, I would think they could get the ip info from your, (or whoevers), isp, much like the RIAA is doing right now, and if they could prove those messages were from you, then yes... I would think they could use 'em against you in court.....
Think about it this way...... If you send a post marked letter to someone, threatening to kill them, it can be used against you. Same thing with email, forum posts could be different, but I wouldn't see how..
Good Luck.. I'd still guess the guy is just blowin' smoke..... Hope I'm right.....