Thank you for using justanswer. I can understand your concern, since your income has dropped so dramatically, however, the IRS gets a report of all income that is connected to your SS#, so they will already know that you have now retired and started collecting Social Security (since there will be no record of any W2, and there will be a record for the Social Security you collect)
As long as your taxable income
is <$9500 ($5800 standard deduction
+ $3700 personal exemption) if you're single, or < $19,100 ($11,600 standard deduction + $3700 x 2 for personal exemptions
for yourself and your spouse), then you do not need to file a Federal 1040.
There are of course, some reasons why you might want to file a tax return. For instance, if you have any Federal tax
with held from your Social Security or your Retirement Pension, you will want to file to get a refund of the withholding
Also, I assume that you have some money invested, and that you will eventually be collecting from a Traditional IRA, or interest/dividends
from non IRA investments, so you will want to plan ahead for the years that you are either required to take an IRA minimum distribution, or for years that you choose to take interest/dividend payments, etc.
You mentioned that you knew that none of your Social Security is taxable this year, so forgive me if I tell you information you already know, but its part of the "planning
If you're single, you add all of your income (pension, interest, etc) +1/2 of your Social Security. If that amount is > $25,000, then you will have taxable Social Security for that year.
If you're married, its the same formula, but the threshold you must be over is $32,000.
For 2011 Filing Requirements, please see below: (you will need to copy & paste this into your web browser)http://www.savingtoinvest.com/2010/12/2011-tax-benefits-to-increase-with-updated-irs-tax-tables-and-income-thresholds.html
I hope this helps