BeeJiveIM connects you to AIM/MobileMe, MSN/Windows Live, Yahoo, GoogleTalk, Facebook, MySpace, ICQ, and Jabber... all at the same time.
In order to use the push notification feature you will need to be running the iPhone OS 3.0 firmware.
For a limited time the application is available at a cost of $9.99.
*Note: Unfortunately, after installing this update I receive a pin.mismatch error when attempting to log into any account. Not sure if this is an isolated incident or a bug in the update.
*Update: To fix this error remove BeeJiveIM from the iPhone. Then do a sync with BeeJiveIM manually deselected from the application sync list. Then do a second sync with BeeJiveIM reselected. You will need to reenter your account info once the application is reinstalled.