Some years ago, I had a recruiter seeking Help Desk work for me.  At
the time, I worked for a company with international offices.  I won't
mince words - the most unpleasant part of the job was supporting the
foreign offices over the phone.  What a pain in the butt to talk
people through a long procedure when their level of computer
proficiency is low, and English is their second language.
 Not wanting to repeat that (and being relatively happy otherwise at
the present job - it's not like I was desperate to leave and had an
"any port in a storm" mentality), whenever the recruiter told me about
a job opening with a large firm I asked if giving phone support to
overseas offices was part of the job description.
 "I can't ask them that.  It's illegal."  She would reply.
 Let's disregard for a moment the fact that what I wanted to know
would most likely have been on the job req.  Was she right about it
being an illegal
question to ask?  It's not like I said, "Please ask the HR department
if only Caucasian people work at this firm."  I have to emphasize that
I am not a Xenophobe or a racist, I simply found it draining to
support overseas users and didn't want to repeat that experience.

According to the list at:
http://jobsearchtech.about.com/library/weekly/aa022403.htm

"National Origin" is one of the things that it is illegal to
discriminate against.  However, note it is not illegal to ask about
this and other issues, it is only illegal to deny employment solely
based on them.

Your case is tricky, because on the one hand, it does seem like the
question is very relevant to the job descrition and required duties. 
On the other hand, you are essentially discriminating based on
national origin.

In closing, I just wanted to say that I have some experience with
providing tech support over the phone, and that was plenty difficult
when the person on the other end was a native english speaker!  So
while I would not want to deny tech support to people in other
countries, I sympathize with your desire to not be the person
providing it.

As you essentially indicated, most employment laws are about
discriminating against a potential employee based on national origin,
not a potential employer, as far as I know.
 In addition, what I really did was ask the recruiter to check the job
req a little more carefully to see if supporting international offices
was part of the job.  She blew it up a bit out of proportion, IMHO.
 I agree that this can be a delicate question, but I still think that
simply saying "am I required to support overseas offices" *should*
really be a fairly innocuous question.  In fact, it's a smart one
since having to do so usually means that the tech support person must
wear a pager periodically when those offices have an emergency not
coinciding with the operational hours of the mainland office.