Okay, so, I'm going to start off by saying that, yes, I'm new here, but no stranger to BlackBerry handsets m'self. However, I have learned a great deal from these forums, and in turn would like to contribute something back, here.
Research in Motion's Yahoo! Messenger client (located at http://www.blackberry.com/go/yahoo
) requires a specific Y!M service book to be able to function on any given handset. Now, as I'm sure some of you are well-aware, Cingular Wireless sadly does and will not currently provision the Y!M service book (BBFS2) when downloading/installing Y!M. However, other service providers will, and I have taken the liberty of seperating into a Backup/Restore file the service book for Y!M into a nice, easy Backup/Restore File, which can be loaded up right into the device via the Desktop Manager Backup/Restore (I'm sorry, no desktop solutions for Mac users yet, but stay tuned!). Just make sure that you remember that this is going to delete all your current service books, so you 'll need to manually register via the Host Routing Table editor in Settings > Options/Advanced Options to get your books back. Also, you'll need to log into your BIS account and resend the service books from there, as well as regenerate any encryption keys if you're using the Desktop Redirector or are on a BES (don't forget to have your friendly neighborhood BES admin create an IT policy enabling Y!M traffic, too).
Now, the other way this can be done involves inserting another GSM carrier SIM card (such as T-Mobile, for example) and downloading/installing Y!M from http://www.blackberry.com/go/yahoo
from there. At that point, you should check your service books and make sure that you've got "BBFS2 [YHO]" listed, then switch back the SIM card to your Cingular SIM (your handset must be unlocked to proceed, though).
Basically, there are just a few steps to go through here:
#1: Download the .ipd backup file here:
#2: Restore the file to your handset using Desktop Manager (see https://cingular.atgnow.com/cng/tutorials/KB4717.html
#3: Manually register your handset with the RIM network via the HRT editor (see https://cingular.atgnow.com/cng/tutorials/KB49643.html
#4: Log into your BIS account and re-send BIS service books:
BIS 1.8 (from PC) - https://cingular.atgnow.com/cng/tutorials/KB2355.html
BIS 2.0 (from PC) - https://cingular.atgnow.com/cng/tutorials/KB64052.html
BIS 2.0 (from handset) - https://cingular.atgnow.com/cng/tutorials/KB64276.html
#5: If applicable, regenerate encryption keys for Desktop Manager or BES. If you're on a BES, again, make sure your BES admin can create a policy allowing Y!M traffic.
#6: One more thing: if you find that the service does not work right from the install, try doing a hard reset (pulling/reinserting battery).
I know it's not very organized, but I hope you all find this useful. Good luck, and happy messaging. =)