I'm hoping that 'Dragon Riding' is misleading and doesn't actually indicate that we'll be able to do it whenever we want.
Bethesda needs to take a page from From software and Dark Souls when it comes to dealing with player feedback that doesn't mesh well with the game world: When Dark Souls players begged for Voice Chat, From added it in the DLC... in the form of 5 wooden orbs that only say one word and must be used from the inventory. It was hilariously trollish.
if a non Dragonborn character can't visit the island then that destroys about all replay value
It would svck if you couldn't swim over, ala Morrowind.