Aim is working now on OS 5 officially as are all the RIM IM apps.
Get it from appworl or try BlackBerry Wallpapers, Ringtones, and More from your bb browser or mobile.blackberry.com and follow the instant messaging link ( also from your bb browser )
Hope this helps
Bodisaffa
Posted from my CrackBerry at wapforums.crackberry.com