Both parties install the iPhone app and you can send unlimited IM's to each other, but the difference is you can set the Instant message to automatically delete itself on both your phone and the receivers phone after a certain amount of time.
You can read about it more here:
You have to verify your cellphone number once to verify the service. Hopefully this verification service works on vodafone, telecom, 2degrees, and telstra.
I wonder if people will use this app when they are texting and driving? They could send an IM to someone while they're driving and set the IM message expire time for 1 hour so after 1 hour the IM will be deleted from their phone and the receivers phone and also from TigerText servers. Wonder how the police would prove texting and driving with this. lol