I fail to understand why you would want to do this. I can only assume its because you saw the $99 price tag on the iphone 3G, and think you can use that instead of 250 or whatever for an itouch.
Rest assured, this will not work.
The 99 dollar price tag assumes that you are either an eligible device upgrade or a new line of service - either way it involves signing a 2 year contract, which means you're going to be paying a monthly service charge (or 150 bucks to cancel your line of service).
Realistically, without contract, I believe the 3G is now $299, with the new 3GS going as high as 599/699. without contract, obviously.
Also the response by 'Steve' is 100% inaccurate. The iphone can absolutely be unlocked and used on other carriers (tmobile in the US, and pretty much any other SIM network out there...Steve, if you're reading this, go to yellowsn0w.com and quickpwn.com and read up)