MP5 or MPEG-5 is non-existent, undefined. Similar to MPEG-6. (MPEG-3 is a different story altogether).
Whatever it is these players you say are boasting of, its not related to the actual MPEG standards (Which has only MPEGs 1, 2, 4, 7 and 21). It could, however, be some altered MPEG-2 compression technique, or related to DivX encoding, but it surely isn't standard and you probably won't get much media for it.
All these hopes hold only if its not some foolish company boasting away with numbers.