Requirements for TCAS equipage in U.S. is for all Part 121 (airlines and larger commuter aircraft) carrying pax for $. Ther e are some requirements for freighters too, I believe. TCAS uses the transponder provided information (including Mode C if the a/c provides it) but TCAS II Version 7 (the current one) has things in it to make use of all kinds of transponders, not just Mode S, including some older transponders that have some quirky logic in them.
Mode S has been in airliner and higher end GA aircraft in the U.S. for over 10 years. The reason for equipage is requirements in some terminal areas because the "regular" transponders couldn't handle the number of target replies in the time available (gross oversimplification provided gratis here)
The ground sensor for Mode S will take inputs from all kinds of transponders. There is a data link feature in the Mode S that was on it's way to being used in the U.S. a few years ago but the cost of building a system using it and getting everybody to equip and who knows what other factors was apparantly a "too hard" but the capability is still there. There are some people looking to use this spectrum and data link for surveillance and communications but there are some competing methods and it is not completely certain which data link will be used. Check out ICAO SICAS Pannel and Eurocontrol web sites.