I have noticed a subtle but fundamental difference in the usage of this word in British and American English.
In British English, if something happens momentarily, it is understood to take place for a very short period of time. For instance, when cartoon characters run off of the edge of cliffs, they are generally suspended momentarily in mid-air, frantically running in place, before noticing their predicament and promptly falling.
In American English, if something is going to happen momentarily, that means it is going to happen within a very short period of time. For instance, while on hold waiting for Dell support to answer my call, a recorded voice repeatedly informed me that, "a member of our support team will be with you momentarily", which to me kept conjuring up visions of someone answering the phone, saying hello, and then immediately hanging up.
Webster 1913 bears out only the British usage, which leads me to suppose that the current American usage is relatively recent. I'll be confirming that momentarily.