This application can be downloaded for iPhone AND for Blackberry. Currently they're working on more smartphone applications, so let's hope it works.
Basically this is a FREE SMS service. Install the app and you can send a text to people, as long as you're connected to the internet.
Push notifications let you know when you got a new text, and you can access all chats:
Something that is new are statuses. If your other contacts have WhatsApp installed, you can choose a status and they'll see that status.
You can also check how many messages you have sent/received, how many bytes sent and received etc
For those interested: WhatsApp Website
Appstore: €0.79 / $0.99