is TCAS required

[500 above]

Not in the United States, or the NAT and PAC oceanic areas. However, Eurocontrol has mandated the carriage of ACAS II in European airspace for all aircraft over 15,000 kg (33,000 lbs) and as of 2005, aircraft 12,500 lbs and over are required to have ACAS II. Operation in RVSM airspace (excluding Europe) does not include a TCAS requirement. It does, however, require MOPS 7 software for TCAS II-equipped aircraft. That is TCAS version 7.0 software.
TCAS version 7.1 will soon be offered as an upgrade by all of the major TCAS manufacturers, and also makes two important safety enhancements. Version 7.1 changes the current TCAS II aural warning from "Adjust Vertical Speed, Adjust" to "Level Off, Level Off." It also corrects missed and late TCAS reversals. TCAS reversals were introduced in TCAS version 7.0 to adapt to changing situations where the original sense had clearly become the wrong thing to do, in particular the situation when one of the pilots decides not to follow the Resolution Advisory (RA), or is instructed by ATC to perform a particular maneuver. The solution in Change 7.1 introduces improvements to the current reversal logic to address late issuance of reversal RAs and potential failures to initiate reversal RAs.
TCAS version 7.1 also makes four other minor enhancements to the system. Version 7.1 corrects an issue when descending through 1,000 ft AGL. 7.1 also modifies the "Datalink Capability Report" (the TCAS status report sent by the TCAS processor to the Mode S transponder) to tell the systems that the TCAS processor is Hybrid Surveillance-capable. And 7.1 also allows for the transmission of the TCAS processor part number and software level, and corrects TCAS multi-aircraft logic issues which reduces the risk of "close-encounters" of multiple aircraft in RVSM airspace.
The enhancements introduced in TCAS version 7.1 proved to be significant enough to warrant mandates by both the International Civil Aviation Organization (ICAO), and the European Aviation Safety Agency. ICAO has mandated TCAS version 7.1 by January 1, 2014 for forward fit aircraft, and January 1, 2017 for retrofit aircraft. EASA has stepped their time-table up by comparison with forward fit aircraft due by March 1, 2012, and retrofit aircraft by March 1, 2014.

So, no. It's not required to be operable - just on board. MEL's obviously to be considered...

For the Americans:

Part 91 Appendix G, Section 2, paragraph (g) is the only regulatory language that relates TCAS to RVSM operations. It states that:
"...if you operate an aircraft that is equipped with TCAS II in RVSM airspace, it must be a TCAS II that meets TSO C-119b (Version 7.0), or a later version, unless otherwise authorized by the Administrator."