Here's a picture of what some of the team buses look like. Second from end is the Iranian bus - no security worries there. Here's a second picture that includes the US bus (second from left). Note how it also doesn't have USA where the other nations' names are.
I've told my family that I'm boycotting the World Cup if this decision isn't changed.
Write, call or fax the US Soccer Federation to complain.
U.S. Soccer Federation
1801 S. Prairie Ave.
Chicago, IL 60616
Phone: (312) 808-1300
Fax: (312) 808-1301
Or you can e-mail them using this page.
UPDATE: I sent a fax using tpc.int service (be sure to use proper format in fax number +1-312-808-1301.)