One underreported provision of Google’s billion-dollar deal with AOL is that AOL will make Google Talk interoperable with AOL Instant Messenger. Point four of the press release:
Google Talk Instant Messaging Software to Communicate with AIM users
And a little further down:
Enabling Google Talk and AIM instant messaging users to communicate with each other, provided certain conditions are met;
This is game-changing, like several portions of the agreement. This sets up a straight up two-on-two battle, with Yahoo & Windows Live Messengers interoperable on one side, and AIM/Google Talk on the other. Considering AIM is the most popular client on the market, and Google Talk is the lightest client on the market, this alliance creates a powerhouse protocol that Yahoo and Microsoft, with their similar products, may not be able to compete with.
Expect AIM to keep its market share lead for a very long time.