AT&T in the USA has finally enabled MMS on the iPhone 3G & 3GS. Due to reasons I find hard to believe, MMS is not available on the original iPhone. Anyway, to enable this, you have to do a couple of things.
1. Connect to iTunes with your iPhone and click 'Update'. iTunes will tell you that you have a carrier update available.
2. Restart your iPhone.
Now you can go to the 'message' area and you will now have the option to add a picture or video to your text message. You will also see it crop up in other areas, such as 'Contacts', 'Photos', etc. Usually where you have a 'share' option.