It’s definetly blocked by the App store because that would allow to connect to a shared iTunes library. Apple doesn’t want that. Apple wants you to sync it over the wire just like an iPod. *shrug*
But it’s not as easy as that, since there are similiar devices around and there just doesn’t seem to be a half-decent DAAP client implementation out there. I’ve had an Nokia N810 for almost a year and there is no such thing, not even close. Thats surprising, given it’s open nature.