i don't see why marat would be a 'dark horse', seeing as his ranking slide was largely b/c of his injuries last year.
dominik could probably break a few hearts along the way. he's a great hard court and clay court player, since he did make it to the qf at the ao back in '01. and he's very quick - did any1 see his match against xavier at the hopman cup a couple of years ago? that guy can run several marathons and still find his breathe! he hits pretty hard as well, and i reckon he's got one of the best serves and forehands in the game (great topspin!) - so he'll be lethal against players like juan-carlos, who aren't great counter-punchers against power-players.
rafael might stand a chance, but like taylor dent, marat, tommy robredo and rob ginerpi, he's prone to unforced errors! plus i reckon he's weak against all round players like roger, rainer and yevgeny. a 4th rnd finish might not be too far a stretch for him.
i think players like sjeng and paradon are a good bet to go far at the ao. both are good on fast surfaces. xavier, lleyton and carlos could make it to the semis as well