Didn't Ronald 'Raygun' get an honourary knighthood?
EDIT: Yeah, he did.. http://query.nytimes.com/gst/fullpage.html?res=950DE7D6123AF936A25755C0A96F948260
I think foreigners like old Raygun here cop the 'Honorary Knight Grand Cross of the Most Honorable Order of the Bath' and, though they don't get to be addressed as Sir, they do get the added excitement of putting the initials G. C. B after their name!
Each order of chivalry has different rules, no Yank could be in the highest order of the Garter, but there are plenty of others they could.