My first guess was plane because they kept saying how far away Unova is from everywhere else. And I don't mean we'll go to Kanto/Johto, just that it's possible to go in and out of Unova which lends credit to the second map speculation.The airport in Mistralton? The one that B/W explains is used for cargo flights coming/going from the city? Of course, that could be changed in B2/W2 and it actually does passenger flights now, but I wouldn't jump to conclusions too quickly here--Giovanni being in Unova doesn't necessarily mean we'll go to Kanto/Johto, the same way that one Rocket grunt in Castelia City didn't mean anything. And besides... who says he arrived by air and not by sea?