Accountants: In what year is the income taxable when an option is written in December, and expires in January?

For this example, I write a January 2011 put in December 2010. I received the premium in 2010. The option expires in 2011. Regardless of whether it is in, or out, of the money at expiration, I received income in 2010. I believe that income is taxable in 2010 as a short term gain. My friend disagrees with me. Can anyone help us understand the correct answer?