Similliar in nature to the RVSM exemption offered a few years back. Remember it is at the "discretion" of ATC. Don't count on it always being available. There are so many inexpensive ADS-B solutions available.
 
The irony is that even though your aircraft is required to be ADS-B compliant by Jan 1st, ATC is not going to be relying on ADS-B anytime soon. You still have to maintain your mode C transponder and there's no telling when the ATC side is actually going to make the switch. They're going to be using mode C for quite some time to come. Could be many years before they switch....