I got this info off of 3 different message boards, so take it how you want to.
Starting point: Teams who spotted in both Boston (4 teams) and Chicago (2 teams.) Some have speculated that either one of both of these cities is a connection point from another city that might not have an international terminal. Someone who lives in Chicago said they got access to a film permit used, and Buckingham Fountain, the Sears Tower, and O'Hare Airport were shown as given permission to film there. I read it, although I can't remember where, but one team spotted in Chicago was an all-female team (no info on how old or any description.)
First destination: Iceland. I take this with a grain of salt until I hear more. One person on one board said they have a friend who lives in the capital, and was going to pick her father up, and saw teams running out of the airport and cars with the yellow/red flag. Didn't give any other info. Then a person from realiiity.com said they were actually on a flight from Boston to Iceland with 4 teams (described as an all-male team, a blonde mother daughter type team, a black couple, and a young couple.) This person also said that TES Harry Smith and the weather guy were spotted as well (someone suggested maybe they're doing a behind the scenes similar to what they've done with Survivor in the past.) God forbid they are actually racers. I don't really watch TES so are Harry and the weather gut missing?
Also, another potentional cast spoiler (not big one); about two weeks ago, film permits were taken out in the Miami/Ft. Lauderdale area, so a team may possibly be from there and they were filiming them for the credits.
So this info is all pretty much second and third hand, as I said above, take it how you want it.