http://thenextweb.com/2009/09/29/ping-instant-messenger-exclusively-iphone-users/
I tried this app for the first time last night. Not really impressed. Interface is clunky and there seems to be some major bugs. Like I sent a message to my brother and he did the same. Worked fine then after two messages, it got crazy. We sent a third message and it came in twice on each phone and double popup notifications and dings. Mine had the same issue but decided to notify me a third time, a minute after the double notifications. Then a badge appears to tell me there is a new message when there is not. Going in the app and exiting still leaves the badge.
It was promptly deleted. Again, I don't get why this is needed. There are a host of other applications out there that people already use and connect to. Aim, Yahoo, MSN, Google, etc.