If you use an AT&T Blackberry and are getting the message "Service not Enabled for this handset" message do the following:

PLEASE BACKUP YOUR DEVICE FIRST.

- Download the Yahoo Messenger 2.0 service book (NOT THE APPLICATION) from http://www.box.net/shared/aetu7mfqip

- Open Desktop Manager and restore the above file to your device.

- Manually register your handset with the network via:

Options > Advanced Options > Host Routing Tables > Menu > Register Now

- Load your own backup back to the device.

- Log into your BIS account (www.qtel.blackberry.com), goto Help (on left side) and re-send service books

- Download Yahoo Messenger from mobile.blackberry.com

It should work fine even with FREE Yahoo.