In order to use the iPhone you need to have either an AT&T sim card with an account or in the case of an unlocked iPhone any other GSM sim card that is on an active account. I am not aware of any possibility to use the iPhone as a real phone over WiFi at this time.
When you say unlocked iPhone you are simply referring to the phone no longer being carrier dependent. Thus being able to use any other GSM carrier instead of AT&T for example.
Already have one. But I want the camera, and the option to get a plan if I so please later on down the road. And I forgot about the SIP app thats out if you jailbreak it. That way I could use it over my home network. And then I could find a free SMS program, and there are plenty to choose from, and bam. Full iPhone, without a AT&T plan.