Encoding cannot be discovered with this technique. It only detects the energy. All phase information is lost.

UWB transmitters can be detected simply by correlating between multiple receivers. And if they're stationary you can even use the correlation delays to triangulate it's position. Basically if it emits energy and you have enough time/receivers you can find it.

Remember that their cloud solution is somewhat questionable. http://www.forbes.com/sites/kellyclay/2012/07/19/is-microsoft-spying-on-skydrive-users/ If you enable the cloud sharing you risk getting your MS account banned if you:
"depicts nudity of any sort including full or partial human nudity or nudity in non-human forms such as cartoons, fantasy art or manga. advocates, or expresses pornography, obscenity, vulgarity, profanity, hatred, bigotry, racism, or gratuitous violence."

As an example if you're a manga artist and you use skydrive you just might find yourself banned losing access to everything. If you write a naughty story in Word with sync on you are violating their terms of service.

I am aware that it is most likely very rare that there would be massive bans. However the fact that they have already banned some users of Skydrive simply due to the content of their private files indicates that they will probably do so in the future.

Actually it's not just "Similar OS but with more features". WP8 is a completely new OS with completely new API. You need to do everything differently. WP8 uses the WinRT API. It's like going from pure java android (ndk not allowed) into iOS:

Accepting payment is going to be a hassle. You cannot accept any credit cards (I doubt there is any credit card in existence which does not work in USA yet works in other countries) nor can you use banks to save your money because they're almost certainly 'tainted' by VISA/MasterCard.

I bet you can go to 25m too if you just average the signal enough (use multiple cycles). The chirp and averaging both are there just to increase the signal to noise ratio. I'd say the best way is just to use all the available bandwidth in the device (range resolution is determined by bandwidth), not limiting oneself to just audible or inaudible parts of the spectrum.