In 2008 I was an F-1 student, but was present in the USA only 27 days (I kept my student status at all times).
I was present in the USA 228 days in 2007, 105 days in 2006, 250 days in 2005.

Should I file as a resident or as a non-resident?

Thank you.

2008 -- nonresident tax return with Form 8843.
Read: Your U.S. Tax Return: U.S. Tax Filing Requirements for Non-Residents (http://taxipay.blogspot.com/2008/03/us-tax-filing-requirements-for-non.html)