Do I have to file a tax return on my Social Security income?

I received a form notice 703 stating that I "MAY" not have to file taxes on my 2008 SSA income of $8,691.40? This is my only source of income. How do I find out if I file taxes, knowing my income is not taxable. The letter states I "May "not have to file. I have spent hours on the phone with IRS and I only wait and I finally have to hang up. Thanks