According to some information from an internal AT&T employee memo posted on the iPhone Atlas site, the iPhone 3G may actually include MMS support. This multimedia messaging support would allow you to attach photos and maybe short videos and audio to your text messages, just like you can do now on any number of feature and smartphone devices today. It is nothing new in the mobile phone world, but it is something people have been asking for on the Apple iPhone.
I never used MMS much, until my teenage daughters started using cell phones and now we shoot quick photos and video and send them back and forth quite a bit. The nice thing about MMS is that you do not need a specific wireless data plan to use this functionality since the content is transferred over the same network settings as text messages. And with our grandfathered unlimited text message family plan we get this functionality on all of our phones for a low cost of only US$10/month.
We still don't know if there will be any special text/MMS messaging plans with the iPhone 3G yet since the current iPhone comes with 200 text messages with the data plan. AT&T may use their same text message/MMS data rates or maybe they will offer some special plan for new iPhone customers.