CPB - Good Post, just to clarify a couple of points.
At present TCAS and Mode S ground stations operate completely independently. A ground station would see TCAS squitter or air to air surveillance as interference and visa versa.
Mode S ground stations do not use the squitter data but transmit an All Call interrogation to find out who's out there which will also include its interrogator identity code. This then gets included in the transponder reply. All Mode S interrogations and replies are parity encoded so that the data is only used if it contains the expected aircraft address or interrogator code.
The way that Mode A/C systems reject interference is to interrogate a number of times and only believe the reply if it gets the same information more than once (There's also some clever decoders that correlate replies on amplitude and angle of arrival.)
At the moment the only link beween TCAS and ground based Mode S is that the transponder will generate a message to the ground station that contains the RA information if one is generated. This is of limited use to co-ordinate actions, it wouldn't arrive in time, but could be used to clearly identify the aircraft concerned on the display. At present there's not many operational Mode S stations outside the US and even these don't use this feature. However, most of Europe will be updating to Mode S in the next couple of years.
Lots more proposed for Mode S in the future, see the Eurocontrol web-site for info on 'Enhanced Suveillance' and ADS-B using 1090 extended squitter.