Having announced its iPhone-compatible sleeve earlier in the year, Sprint MVNO FreedomPop is now circling back for a second version that's compatible with the iPod touch. Aftermarket cases that graft cellular radios to the iPod touch are nothing new — Sprint launched ZTE's Peel in 2010, for instance — but what makes FreedomPop unique is its business model: everyone starts with half a gigabyte of monthly WiMAX service for free and can earn up to 5GB through a combination of friend referrals (10MB each) and participation in "partner offers and promotions." As with similar devices, the iPod 4G Sleeve works through a Wi-Fi connection and can also function as a mobile hotspot that allows up to eight devices to connect to it (though with only 500MB free to start, you might be reluctant to let too many friends hop onto the signal). Like the iPhone sleeve, the new model isn't yet available, but FreedomPop is accepting pre-orders at its site starting today for $99.