Folks look. . . . If you are on Cingular the only applications from RIM that officially work w/out any mods are Google Talk and BB Messenger (although BBM is not officially supported by Cingular's customer service but it still works on RIM's network).
The other IM clients that RIM released (accidentally) about a month ago don't work either. However the RIM's Yahoo application does not work on Cingular's network either as originally installed from RIM (as spega indicated, read the fine print as it doesn't work on Cingular/AT&T network).
If you do want to have Yahoo working on your BB and you are a Cingular subscriber please go to this thread for detailed instructions on how to make it happen:
http://www.blackberryforums.com/blac...-handsets.html
Cheers and best of luck!