The Brits and Europeans are waaaaay far
ahead of the FAA on this:

http://www.pprune.org/private-
flying/579061-ads-b-live-experience-ads-
b-fly-near-you.html

"NATS has supported the connection of
»uncertified!« [my emphasis] GPS to Mode
S transponders, to see how good the ADS-
B data is and has concluded together with
the CAA that it is acceptable for use in the
UK. The LAA has now taken over the
approval process and an increasing
number of aircraft are becoming
equipped. This equipment is interoperable
with “classical” ADS-B."