I believe that the change from the use of M to mean 1000x to the use of K is part of a general trend to use metric prefixes. If CM is circular mil, then is MCM 1000000 circular mils or 1000 circular mils.
The problem is that depending upon where you are coming from, the answer is _clearly_ 1000, unless you come from some place where the answer is _clearly_ 1000000.
-Jon
