Anonymous - no way. A small amount of location tracking + the additional data any agency and many others can easily access will easily identify an individual. I read the UCSB paper referenced by the Whisper CTO - it just said there was a hard problem Whisper was trying to do something about. The paper also said that each user had a permanent GUID. So if I, with my GUID, get on a plane from SFO to (say) Santa Fe on one particular day - the GUID use moving will make it clear I have taken a plane - then the agency (or perhaps my credit card issuer?) will get the candidates for my GUID down to a few hundred at most just from that move and the passenger list (or ticket purchase records). Coupled with my GUID's home city and work city and they probably have me nailed - just like that. Trivial.