I missed this app too. MobileChat is multi SNS client app for iPhone / iPod Touch.

This app can connect to Windows Live Messenger, AIM, .Mac, ICQ, Yahoo! Messenger, Google Talk, and Jabber.

Login process is buggy a little. And it doesn't run in the background.

Push is available for AIM and .Mac account, but I guess you may go bankrupt because it is able to do through SMS.