Remember when the US won the medley relay by four seconds at Worlds? I miss those days. In this era, I can think of six countries with somewhat of a shot to win the event at Worlds. Here are the teams:
USA - we don't know who yet
Australia - Delaney/Stoeckel, Rickard/Sprenger, Lauterstein, Sullivan
Japan - Irie/Koga, Tateishi, Kishida, Fujii
Russia - Vyatchanin/Donets, Geybel/Lakhtyukhov (both 1:00-mid/high), Korotyshkin, Grechin
France - Stravius/Stasiulus, Duboscq, Lefert, Bernard/Bousquet
Brazil (since when?) - Guido (he's put up some quality 50 Backs, still waiting for a good 100), Barbosa (59.12 - 2nd all time), Mangibeira, Cielo
When a country starts getting the swimmers of the various strokes, their medley relay gets good. Unfortunately for the Americans, it happens.
As for Duel in the Pool, the Japanese men seem more primed than the Aussies, so I think they will take out the medley relay. However, the Aussies will be tougher in Rome, but it will be a battle for any of the countries.