I believe that any device thats not running on a PC has to be licensed as hardware, which has a different pricing structure. I think the software platform thing doesn't really matter since all MP3 decoders are "software"; the MP3 spec is impractical to implement as a state machine or hardware pipeline because of the many different features in it (stereo modes, block sizes, frame sizes, etc). I believe the determining factor is how open the platform is (PC verses DAP/DVD player/whatever.
This is from memory though, its been ages since I looked at the license.