A native MMS (Multimedia Messaging Service) application has been released for the iPhone. The 0.1 release only allows sending MMS messages and will not receive them. MMS allows you to send multimedia (such as photos) to others via text-messaging. In it's current incarnation, the iPhone only supports SMS (text messaging).
The application is still an early version and requires some manual setup to work with your provider. More information is available on the author's blog.
Owners of "hacked" iPhones can install it using Installer.app.