An MMS picture from an iPhone 4 is being received as a video on my GSII which plays for 10 seconds.
An MMS video from an iPhone 4 shows up as a generic icon with a play button--pressing it plays only audio.
An Iphone 4 cannot play the MMS videos sent from my GSII.
Video taken with an iPhone 4 is in Quicktime format (.mov) which the GSII can't play without an aftermarket player (Rockbase, etc.). The reason MMS pictures from an iPhone show up as a video is, non-iPhones see it as a slideshow. As for the videos from the GSII: the iPhone doesn't like the 3gpp format.
(*edit* If you send video from the GSII via email rather than MMS, it'll go as the original mp4 which is playable by the iPhone.)