Sigi Schmid had indicated that twenty would travel to Panama and then to Houston. Barring an injury those would be the players who make up the two gameday 18s. While the Seattle Sounders do not release a traveling roster, there are clues. First were a few players posting on twitter and/or facebook - Mike Seamon and Fredy Montero. Then was San Fransisco FC's website talking about Mauro Rosales.
Then the team posted these pics to facebook, this video to their website and the roster is known. Surprise travelers are Seamon, Josh Ford and maybe Nate Jaqua. The Church will be pleased as he's dead center in this photo.
Keepers: Kasey Keller, Ford
Defenders: James Riley, Jeff Parke, Zach Scott, Patrick Ianni, Leo Gonzalez, Tyson Wahl
Midfielders: Alvaro Fernandez, Osvaldo Alonso, Seamon, Erik Friberg, Rosales, Pat Noonan, Lamar Neagle, Servando Carrasco
Forwards: Montero, Roger Levesque, Nate Jaqua, Mike Fucito
What does this all mean? Follow the Sounders FC Facebook feed and as many players as you can on twitter. Reading the opponents website can help as well.