I am with Cingular/AT&T and the carrier seems to be blocking both of these messaging platforms. Any ideas?

It is not that they are blocking them but that they will not send out the service books for these programs. From what I know RIM has not even really released the service books for the Windows Live program yet. I remember awhile back that you could get the service book from a zip file on the website or it might have been abother forum. Not really sure. As for the Yahoo good luck on ever getting AT&T to ever send that service book. If you want these types of IM chats then you are going to have to look at other programs. JiveTalk works great and it is the cheapest program that does alot of different services.

Using RIM's Yahoo! Messenger Client on Cingular Wireless BlackBerry Handsets

Try this guide from this site. It will let you install RIM's client on your Cingular/ATT Pearl. It looks complicated at first, but it takes about 5min tops and is very simple even if you don't have a lot of tech knowledge. Basically:

1. Download the file with the link given
2. Use Desktop Manger to "Restore" (aka load) the file to your Pearl
3. Re-register your handheld on the network (click on an option in your option menu)
4. Log into your BIS/BES account from the link given and click a button (should already know your login.password)

That's pretty much it assuming it goes smoothly (mine did). You may need to do a hard reset or something but not likely.

Yesterday I used the Cingular MediaNet browser on my Pearl, Go to Mail and Messaging link and then click on Mobile IM, the browser will send you straight to the download link. You can download the application from AT&T and you will get the IM Messaging application. you can log into MSN, AIM and Yahoo. It seems to work fine and it's free. I did not download it from Blackberry, downloaded it from Medianet.

This is a new client that is available just today from AT&T. Does not pick up all the attritibutes of yahoo messenger like the Blackberry edition. I like the "real" client better myself. I tried this "OZ" version, did not like it, and had to uninstall it from the handheld. FYI, the Blackberry desktop software provided by AT&T views the RIM yahoo messenger as an unrecognized application and then uninstalls it when you load applications. Then you have to reinstall yahoo.