I'm looking for advice on this problem...

In our current system, we link a person's IP address and username to a user license, thereby allowing some flexibility for how we license, etc...

Switching to the mobile world - as a person moves between one location and another - or switches from WiFi to cell - an IP address or the like would be reassigned and changed. Is there any other 'identifier' to the mobile device that I could key off that would give me a unique signature for that particular device?

Thoughts? Advice on how others have handled this type of 'recognition'?

Aaron