Wierd that every other messenger is unlocked and not yahoo.

I think the problem with the Yahoo messenger is that AT&T doesn't provide the service books, because the app installs. If you install the service book then reregister your device under routing tables, then install the program it should work. Other people have gotten it to work. The service book can be found here.
YMESSENGERv2_servicebooks.ipd - File Shared from Box.net - Free Online File Storage

On a side note I would recommend BeeJives, JiveTalk which is an all in one chat application. The majority of people here use and recommend it.

Att allows live and google messenger to be installed so that can't be reason for not allowing yahoo......

I don't know then, because if you install the service books first then the app it works for me.

It works with the service books installed. I have had it for a while.

I just click on that link and dl that service book? Or do I gotta dl to comp and do stuff to it?

Download it then use DM to load it on your BlackBerry

Those apps don't requires special service books like the Yahoo Messenger.

Neither does Jivetalk (which works with Yahoo, among others) -- again, what I'd recommend using. Why wait on IM apps that require service books from your carrier? The BB is data capable using its existing service books.

Wirelessly posted (at&t BlackBerry 8310)

at&t also doesn't allow AIM. I can't even download it from the mobile.blackberry website. I just says that my device doesn't support it.

+1 for Jivetalk, never use anything else.

not sure that they ever will:

I had this same problem, I called AT&T this is what they told me and help me get an IM that works with AIM, MSN, and Yahoo.

go down to your media browser (media Network icon on the bb 8310)
go down to mail and messaging go down to mobile IM, click on it
its called OZ messaging it works awesome... didnt mess with my blackberry at all..

again
media browser/mail and messaging/mobile IM then download login

sometime i find that i have to refresh my contact list after login but no big deal... and its FREE

I believe that OZ uses text instead of data. Which is fine if you have an unlimited texting plan.

Here is an update I read on Berryreview. It seems that yahoo messenger is now available on AT&T without hacks.

AT&T Now Supports Windows Live Messenger & Yahoo Messenger! No Hacks Necessary | BerryReview.com »

There are comments following the article that still say it doesn't work...

OZ uses both data and text. It uses data to log in then text for the convo.

i tried to download the service book from the link you provided. but i was not able to sent it to my blackberry bold. how do i get it on the blackberry ? i tried thru desktop manager but it would not recognize from my desktop so i cant transfer it to the black berry......any help or suggestions greatly appreciated...

Thanks a bunch in advance

Any help on how to download OZ to my BB 8310, w/ AT&T? Help! I am new to this. Thanks.

..... i have had it working fine on my 8310 OS 4.2 for the past few weeks, and even since upgrading to 4.5 still no issues. saved me from buying beejive.