New Coming Apple Peel T288 for iPod Touch 4 (IPT4) (Better than Yosion 520 II)
I'm using a t-mobile SIM card and so far it's working flawlessly.  Note that you MUST jailbreak your iPod in order for the peel to work.  Unfortunately, there is no untethered jailbreak for iOS 4.3.5 as of this writing.  That means that anytime you have to reboot your iPod or if it looses power completely, then you must connect to a PC in order to boot your iPod. Other than that, this is the simplest way to own an iPhone that can use any carrier.