There are lot of issues to consider. Privacy can be one, my feeling is that energy requirements will be the biggest restriction. Most mobile systems are designed to be always ON. Also Any manufacturer of the device will need to put in usb port and charger, buttons and leds. Get it certified by FCC. have a water proof enclosure that is also light weight. the list of requirements of a commercial device are much different than that of a research device.
Also, as far as I am aware there is restriction in the US on putting a GSM device on an airborne platform. (this could be a bird or a balloon or RC plane etc).
I know people have been putting these on birds but that does not mean its legal to do so.