Just adding that i too have this type of mouse lag. ArmA always felt a bit "creamy" to me. Some areas its better and some "creamier". Its not because of lag (well sort of ofcourse), but other games where lag/chop/low fps takes effect i dont get the same slowdown of the mouse. I dont know what it is. Maybe "mouse smooth" as some say here. Maybe an option for it (if there even is any smoother) would eliminate the problem.
BIS, please take a peek at it when you got some time over.