You are correct about iTunes and the original iPhone.
Now back to your original question: Yes you can give your current iPhone 3G to someone and they can use it without the iPhone 3G data plan. They will not be able to use Visual Voicemail.
If they don't have a data plan on their line then they may be charged on a Pay Per Use (PPU) fee for any data that the iPhone 3G uses. This can be prevented by having a WIX Block placed on the line. Just have yoru fried call Customer Service before they move the SIM and ask to have all "internet" data blocked. The AT&T term is a WIX Block and it will only block data but not SMS Txt before you ask, so your friend can still text.
You can test this today, just put your friends SIM in your iPhone 3G and it will work. There is no need for the iTunes "Activation". That part is required for the original iPhone since the iTunes "Activation" paired the specific SIM with the iPhone. That's why the other poster indicated that if your iPhone 3G was unlocked then you had no worries about SIM usage or Activation. In your case with the iPhone 3G unlocking is really only an issue if your friend does not have AT&T and for example wants to use the iPhone 3G on T-Mobile.
AT&T will know that the iPhone 3G is being used on your friends account, the phone will turn them in everytime it connects to the cell network. Currently, AT&T is sending SMS Text messages to iPhones that are using an invalid data plan. There have been several reports that AT&T will begin to make forced changes to the data plans, as of today this is still just a rumor until someone posts a copy of a bill. AT&T can and does have the legal right based on your contract to require the proper iPhone data plan on every iPhone. While I don't agree that you should be forced to have a data plan if you don't need it, I can understand AT&T wanting you to pay for what you use. Some individuals are using the $15 MedaNet plans on the iPhone 3G and I don't think AT&T likes losing the Revenue.
Dave