I DL'd and installed Yahoo Messenger on my BB. I have a Pearl on Cingular service. After the install routine it completes by rebooting. Once I enter my user name and password I get a message:
"Please visit w3 (truncated due to post limitation).blackberry.com/go/yahoo to activate your handheld for Yahoo Messenger"
Now, I've read the posts stating it doesn't work on Cingular but the more recent posts state that they now allow it...