I know you will laugh at me.. But I have been calling from last 4 days to Cingular and to see if my wap /APN settings are ok so that my yahoo messanger can work..
So this Mr. Data Support Specialist at 866-490-2666 goes thru all kind of info and then he asks me if Blackberry is a completely different company than RIM?
I was born in the night but not yesterday!!
I just did not have the simple answer.
I have decided to return my BB8700 .

RIM's Yahoo Messenger does not yet work on Cingular BlackBerries.

Try a third party client:
Blackberry Instant Messaging FAQ
Information about how to set up instant messaging on a BlackBerry, for chatting on the MSN/ICQ/AIM/Yahoo networks. Gai the added convenience of always-on instant messaging, on top of always-on email!