@
Buffmaster
Try this (I haven't tested so I can't verify if it works)
SMS-Chat. Windows Mobile SMS Messenger
I've see a few forums talking about being able to chat via SMS by sending it to a specific number (like Facebook) but I haven't been able to find anything more than a mention of it, no details. If you have time you dig deeper into that method too.
*EDIT: Sorry Buff, didn't realize that app is $15 and it only threads your text convos, nothing to do with IM. Hopefully you're able to find an answer. I'll keep an eye out and let you know if I find anything.