The Only Trusted System is a dead and disassembled system. And even then, you'll want to take a thermite grenade to the hard drive.
From [the Security Enhanced Linux] page:
The phrase "Trusted Operating System" generally refers to an operating system that provides sufficient support for multilevel security and evidence of correctness to meet a particular set of government requirements.
The traditional definition for a TrustedSystem comes from the [JargonFile:OrangeBook Orange Book], which is [available here]. (The CommonCriteria is now the standard to judge trusted systems.)
Another definition is one of need, rather than capability. "A Trusted computer is one which can break the security model", says the [TCPA/Palladium FAQ]. The elements of a TrustedSystem are there to make it more difficult to break the security policy.