If you are talking about the standard encoding of .mp4 video
(http://en.wikipedia.org/wiki/H.264/MPEG-4_AVC) then yes, PPT 2010 (beta at
least) will import and use directly as I just tried with a .mp4 video I
downloaded from the internet. There are some versions of .mp4 that will
not permit it's inclusion into Office packages due to licensing.
It appears that you also have full control of the video just as you would if
you used a .wmv file.