You've probably gotten all your answers based on the other comments. MM or mm should (if at all) only be used within finance. There is enough confusion on units already, that this can't possibly contribute to the better.
A few arguments as to why this is such a lousy notation:
* m and M already exist in the International System of Units, which is used by pretty much everybody else (http://en.wikipedia.org/wiki/File:Non-Metric_User.svg). 'm' is either milli (10^-3) or meters, depending on the context. And M is Mega, or the number in question: one million.
* If the reason is to use roman numerals. M, for mille, means 1000. However, MM would mean two thousand, the same way XXII is 22.
* "Million" isn't ambiguous, unlike "billion", which either means 10^9 or 10^12.
A few arguments as to why this is such a lousy notation:
* m and M already exist in the International System of Units, which is used by pretty much everybody else (http://en.wikipedia.org/wiki/File:Non-Metric_User.svg). 'm' is either milli (10^-3) or meters, depending on the context. And M is Mega, or the number in question: one million.
* If the reason is to use roman numerals. M, for mille, means 1000. However, MM would mean two thousand, the same way XXII is 22.
* "Million" isn't ambiguous, unlike "billion", which either means 10^9 or 10^12.