Bullshit.
Every American learns about metric prefixes in school. We know the difference between a milliliter and a liter, and the difference between a millimeter (about 25 to an inch), a meter (about a yard) and a kilometer (about half a mile).
We use M and B and T when talking about large amounts of money (million and billion and trillions), while they are not the same as the metric abbreviations it still works the same. We also state memory amounts for computers using a sort of modified metric prefixing system.
Just use millibits, mB, and people will get used to it.