This Ping app by the guy who made PushGmail is a mix between text messaging and instant messaging. What you need to know is that it's free and it's fast.
Ping is designed to look like the SMS app, and behaves much the same way. The messages you send arrive at the recipient's screen instantly—similar to IMs—but also pop up with a Push Notification if you're not actively using the phone.
It's basically free SMS for you to everyone who has an iPhone or iPod Touch (over Wi-Fi) as long as you can convince them to sign up for Ping. Even if you can't, the ones you do switch over might be enough for you to downgrade your SMS plan one notch to save some money.
In the end, this may either be a really useful SMS alternative for iPhones, or a service that's not different enough from SMS or IM that people will use it. It's too early to say. [Ping on iTunes via Gear Live]