Though Yahoo Messenger may not be available for the iPhone anymore you can use Beejive http://www.beejive.com/iphone/yahoo-messenger/ I still prefer Yahoo Messenger because it works on the desktop too.
I believe it works natively on Android; on iPhone and Blackberry you have to use a chat app like Beejive, but you can group chat with friends on any platform for the price of a Google Account (and possibly a chat app). Beejive and others also work with multiple IM networks such as, AIM, MSN, Yahoo, Facebook, Windows Live, and Google Talk.