The C Apple Peel T288 is a special cover for the iPod Touch 4G.
It plugs into the iPod from it's cable port at the bottom of it, and it has it's own SIM Card slot and battery source. This combined with a Jailbroken iPod Touch 4G with a Cydia pack called "iSky Phone", which will give you the Dial, SMS, GPRS and Radio apps, your iPod Touch becomes an iPhone 4.
I intend to buy this at some point. I hate my crappy little old phone but don't wanna buy an expensive, Brand New iPhone 4/4S.
If this sounds pretty sweet to you, and you don't mind spending $80 to $100 and Jailbraking your iPod, then I would suggest looking into this. There are other Apple Peels out there, but this is the best I've seen.