Macsimumnews provides a tip on how to send and receive unlimited SMS messages from your iPhone (assuming you have an unlimited data plan). The steps are as follows:
1. Create a new email message.
2. In the To: box, type email@example.com. (firstname.lastname@example.org)
3. Type a message and click send.
4. The message will be sent to the cell phone with the number in the To: box
5. All recieved text messages will be sent to you as an email.
They use the free service Teleflip.com to route your email messages to SMS and back. The service is free, but only works in U.S. and Canada.
This means you won't be getting instant notifications of these SMS->email messages since you have to wait until your email updates. This will be far less of an issue once MobileMe is deployed as you will be able to get Push email sent to your iPhone.