how do I use my iPhone as an Ipod Touch without ATT service or a SIM?

how do I use my iPhone as an Ipod Touch without ATT service or a SIM?

I think you need to insert a sim even if the sim is not in service. I could be wrong.

When I switched the service over from my iPhone to the 3GS in June the old one did everything but work as a phone. Internet (on wi-fi) worked, e-mail sent and received, etc....

Simply buy the phone (whether new or used), and enable WiFi. You'll have full function of the iPod section and all data services (email, IM, app store, browser, etc) providing you're within range of a WiFi network.

You do not need a sim card to use these features (with my old Rogers 3G, no sim is required if you're on WiFi).

I can't remember correctly, but if there is NO sim card, I think there's a window that may or may not stay open saying "no sim installed," correct me if I'm wrong.

You could try using a MiFi. That will give you the constant WiFi you need to use the iPod touch or iPhone, and you can choose the carrier. I use Verizon Wireless MiFi, and it works well. I only use the Skype app occasionally for calls, as I also have an Ipevo Skype phone as well. So I have free everything, except for what I pay for my MiFi.
Incase you don't know what MiFi is, it's a mini router the size of a credit card that emits a WiFi signal everywhere you go.

Jailbreak it.