I spent about half the year working in California, although my permanent residence and the company providing my pay check remained in Michigan. I returned home for a long weekend once or twice a month. I spent 3 months in a hotel charging expenses to the company card and filling out expense reports, and the last 3.5 months in an apartment on non-taxable per diem for living expenses. Will I owe taxes to the state of California?

Yes, you must file CA tax return.

If you are present in a state, then any income earned during this period, must be reported to the state.

To your resident state, you must report your worldwide income for the year.

If you live and work is a state but your employer is from another state, then your income is only subject to the state where you live and work. Read: Your U.S. Tax Return: Working or Living in Two or More states (http://taxipay.blogspot.com/2008/06/working-in-two-or-more-states.html)