F-1 and J-1 Students in U.S. more than 5 calendar years:
Your filing status is determined by the "substantial presence test" which is relatively complicated.
Students who have been in the U.S. in F-1 or J-1 visa status for any part of 5 different calendar years before 2006, and were physically present in the U.S. at least 183 days in 2007, are considered Resident Alien for tax purposes.
If you have been in the U.S. more than 5 years but were not physically present in the U.S. for at least 183 days in 2007, you are considered a Nonresident Alien for tax purposes.
Example calculation:
1998 - present in the U.S. 5 days - nonresident alien for tax purposes
1999 - present in the U.S. 102 days - nonresident alien for tax purposes
2004 - present in the U.S. 340 days - nonresident alien for tax purposes
2005 - present in the U.S. 365 days - nonresident alien for tax purposes
2006 - present in the U.S. 30 days - nonresident alien for tax purposes
2007 - present in the U.S. at least 183 days - resident alien for tax purposes
