Apple has released an article detailing the steps required to move from an original iPhone to a new iPhone 3G.
This article explains how to replace an original iPhone with an iPhone 3G using the same carrier. If you follow these steps to backup your original iPhone first, and then restore the backup to your iPhone 3G, your saved SMS messages, email accounts, photos, notes, and other personal settings will be present on your iPhone 3G.
One interesting fact is that in the USA the SIM card from your original iPhone will not be needed. The iPhone 3G will come with a new SIM card and will be programmed with all of your information. In other countries where the original iPhone was sold users will simply transfer their SIM card to the iPhone 3G.
You will need to download the latest version of iTunes (7.7 or later) to set up the new iPhone 3G.