http://www.irs.gov/businesses/small/international/article/0,,id=96352,00.html
Substantial Presence Test
You will be considered a U.S. resident for tax purposes if you meet the substantial presence test for the calendar year. To meet this test, you must be physically present in the United States on at least:
31 days during the current year, and
183 days during the 3-year period that includes the current year and the 2 years immediately before that, counting:
All the days you were present in the current year, and
1/3 of the days you were present in the first year before the current year, and
1/6 of the days you were present in the second year before the current year.
