sunny is correct, xavi began his career as a deep-lying playmaker. he is a volante at heart. frank and pep have just moved him further up the pitch so he can be deadly in the final third (because of the way we play, the system, we don't need someone back there to ensure we play that way).
iniesta was never a DM, though.
busquets is a DM, however. he started as a striker and it was pep who moved him back. he'll become a ball-playing DM, like yaya or xabi, but he's too good at the defensive part of the game to be moved forward IMO.
come 2010, assuming senna is still injured (let's face it his body is giving up)
casillas
ramos, piqué, puyol, capdevilla
xavi, busquets
iniesta, silva
villa, torres
should be spain's first XI, imo. busquets is a better DM than xabi (who isn't up to the task as many recent senna-less games have shown. then busquets gets moved back there as the DM hey presto, spain win 5-0!)