Ok i think i know how to get it. I have 3 ideas.
1]go through whoel game and not use any health
2]go throuh game but dont use health from medics
3]Do somthing in the secret level
It's obviously got something to do with your health - perhaps if you finish the last level with very little health, like 1, or something. Just a stupid idea.
In the original Playstation version of Medal of Honor, there was a gameplay mode that could be unlocked (I do not recall the name of it) in which your health was not healed from mission to mission. If you finished one mission with 32 health, you started the next with 32 health, and so on through the whole game. (I think the same held true for ammo)
Could a game mode like that be the key to the Purple Heart? Maybe. Have those folks who have already nabbed all the medals tried to start a new game? And if so, does a new difficulty setting show up?
Hey, or maybe the Purple Heart was just cut out of the game. That happens. A lot. Monsters on the Half-Life disc that never made it into the game, levels in Medal of Honor, etc...