I agree with Pam. Sit down with a financial advisor and talk about your specific situation. Also, while you may be upside down on your loan, you want to find out for sure. I would be happy to do a Comparative Market Analysis for you. This will tell us about what your home may sell for in todays market. With this information in hand and an idea of what your options are you can make a good decision that you won't regret. A major hit to your credit score may cost you more than you think.
From the time and location of your home purchase you likely owe more than the home is worth. If making the payment is becoming increasingly difficult, speak to your bank about options you may have. Consider a short sale to less the credit score hit. Walking away from a home i.e. letting it go into a foreclosure should be the last possible option.
If the home is comfortable for you and you are able to make the payments I wouln't walk. Prices go up and prices go down, and again they will go up.
I would sit down with your financial person to see exactly where you stand. In order to guide you here more information would be needed.