When I log into my AIM account on my Blackberry 8830 (Sprint) via the client downloaded from blackberry.com/instantmessaging, on other people's buddylists I still appear offline and they can't IM me.. but it seems that I can still IM other people. My settings on my blackberry haven't been touched in any special way, i dont have any one of those weird invisible settigs or anything..
Pull your battery out to reset your device. Check if you still have the same issue. Otherwise I would suggest get a 3rd party software, either IM+ or JiveTalk.
have you tried out JiveTalk as per Catt's suggestion? It really is the best IM client for any mobile device that I've ever seen! Plus they give you a free 30 day trial to see if you'll like it or not ... you should be able to find a link to their website in my signature.