freedompop (Photo credit: sam_churchill)
Basically, the sleeve lets you turn the iPod touch into an iPhone, so calls can be made over the FreedomPop network using softphones like CounterPath's Bria or Skype. Of course, with Bria you'll need an ITSP (Internet Telephony Service Provider) like Phone.com or OnSip. You can also use the client from 8x8 that lets customers talk and connect to their Virtual Office Mobile which is found in the App Store.
Having used the Clearwire network extensively in the past, the speeds are fast, but what's really impressive is the low latency which means not only VoIP will work, but so will video, making neutral carriers like Vidtel an awesome bridge service for those using just the iPod touch.