Sling Media requires you to buy a license for each platform you wish to run. you can NOT "transfer" from one to another. you can replace a device with the same type of device and transfer the licence.
So for example:
Lets say you have a windows mobile phone and then change to another windows mobile phone. this is allowed; however transfering a license from a windows mobile phone to an iPhone is NOT.
What's more frustrating is the iPone versus the iPad app. Upon release they stated that the iPhone app will be playable on the iPad (which it is); however they didn't mention that when the iPad app was actually produced it would require you to buy another license yet again.