from what i hear, from several people, is you get a special ending and a preview of Starcraft 2. so once i get sick of americas army after a while im gonna try for hardmode. btw if you beat it i suggest you watch the credits, they're funny.
I just beat it on hard. It's not a Starcraft 2 preview it's actually orcs and humans with guns and cool looking suits, but there is some protoss guys in it.
im only on the undead campaign cuz i barely go on single player... but i guess ill play alil bit in single when i get bored with bnet(which wont happen)