I seem to recall that this came up here previously and there was some agreement that it is yet another British v. American usage.

Webster's Third New Int'l gives four senses for momentarily:
1) for a moment <only momentarily troubled by such reports>
2) instantly <he groaned, but momentarily recovered his composure>
3) in a moment <we expect his arrival momentarily>
4) in a few minutes <I'll be there momentarily>

I'll try to check the OED when I get a chance, unless someone has it to hand.