Micromat has a utility called Syphone which is a freeware Mac application that lets you view, save and backup your SMS messages.
The iPhone has a limit on the number of SMS messages it can store at one time. If this limit is exceeded, existing conversations must be deleted in order to make room for new messages. Syphone allows you to view and/or save any of these conversations, including deleted messages.
A beta is presently available for download. Requires Mac OS X 10.4 or greater