On Friday, I saw that the Ping! iPhone application was available for free. Ping is an iPhone application that let's you have SMS or instant message conversations with other people with iPhones or iPod Touches. Ping! uses Apple's push notification service, so don't need to leave the application running to keep receiving messages.
The benefit if using push over tradition SMS messages is that it is cheaper, if you only want to talk to people who also have iPhones or iPods. This is also especially true if you have a limited number of text messages per month on your plan. Also for iPod users, since they don't have access to SMS, this may be a viable solution.
I don' think that I will be using this application that often. Since I already uses SMS with people who are not using iPhones, I don't know if it would be worth it to use a separate application for the subset of my contacts that have iPhones and also have Ping! installed.