Here is a quote from an official FAA publication which can be downloaded here.
IFH-FAA-H-8083-15A.pdf - Windows Live SkyDrive
"Some older mechanical Machmeters not driven from
an air data computer use an altitude aneroid inside the
instrument that converts pitot-static pressure into Mach
number. These systems assume that the temperature at any
altitude is standard; therefore, the indicated Mach number is
inaccurate whenever the temperature deviates from standard.
These systems are called indicated Machmeters. Modern
electronic Machmeters use information from an air data
computer system to correct for temperature errors. These
systems display true Mach number."
Is this nonsense?