I've tried every IMS available for my BB7290 and none worked on the Optus network with the exception of IM+ (http://www.shapeservices.com/eng/im/BLACKBERRY/)
Optus blocks the necessary ports (1863, 443) and will not open them.
IM+ has both a BES and WAP version of their software for a one-time fee. Software works on Jabber, Yahoo, ICQ, MSN and AOL.
Using the WAP version of the software, the settings are:
WAP Gateway IP: 220.127.116.11
WAP Gateway Port: 9201
WAP APN: blackberry.wap
I spent quite a bit of time trying to get a client to work. So, I hope this is helpful.