The football, briefcase, etc, is carried by a military aide from one of the five branches of the armed forces. (It is not carried by a member of the secret service, nor soley a Naval officer).
As an interesting side note, the briefcase is made of black leather skin, with a titanium lining. (Well, I guess its got to look good). The carrier of the briefcase is (apparently) armed with a M9 Beretta 9mm, which may not seem like much, but remember, there is usually a secret service detail near by, and they pack a little more punch.