Thunderbird is indeed a great south-western regional school for international business. I have visited the campus (twice) and spoken candidly with the Dean of Admissions.
IMHO the reason why Business Week and US NWR singles out T-bird as the #1 international biz school is because:
1) ~65% of the student body is international
2) proficiency in a foreign language is REQUIRED before graduation
3) the campus is very small and stands alone - not part of a larger institution.
4) the school only offers three concentrations: global management/marketing/finance.
5) it was founded nearly 60 years ago
As much as I am impressed by the school, I just can't understand how it can admit 85% of its applicants and still be labeled #1
Wharton/Lauder has the same requirements and admits less than a fourth of its applicants.