A Chinese company by the name of Yosion has developed a sleeve for the iPod Touch that is manufactured with an Infineon GSM radio and battery pack that enables it to function as a normal iPhone complete with microphone and SMS messaging, though it does not support data access. 

The sleeve features an 800mAh battery and provides 4.5 hours of call time/ 120 hours of standby time with an expected price of $44-$74 with no word on release or GSM band compatibility.