I saw this today at BlackBerryForums.com: Connect Google Talk to AIM, MSN, & Yahoo
Basically, you download PSI (a Jabber client) and connect to Google Talk. Then you can browse to AIM, MSN, and Yahoo networks. After connecting to these you will have to "accept" connections for each network as well as each contact. It's a bit of a hassle if you have a ton of contacts (unlike me). Then close PSI and login to Google Talk - voila!
Do you want to make it even more interesting? Get Google Talk for your BlackBerry and you have a single client for all or most of the IM networks you would need! On your BlackBerry, go to http://www.blackberry.com/GoogleTalk/ to download the application over-the-air.
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