I married a U.S. citizen in October of 2007. I immigrated to the U.S. in December of 2008, and was given a 2-year green card that expires in December of
2010. If I divorce my spouse after December of 2010, what effect will it have on my Permanent Resident status?

If you divorce after the green card conditions are lifted, you will be fine, BUT if you divorce before filing removal of conditions, your green card will be revoked. However, you may file as victim of domestic violence, in case this applies to you.

If you could not make this 2 years term, it's always wise to have immigration attorney take care of this matter for you.