The Congressional Research Service has published Social Security: Calculation and History of Taxing Benefits (RL32552).  Here is part of the Summary:

Preliminary data for tax year 2002, show that 10.8 million income tax returns had taxable Social Security benefits, and that these returns included $94.7 billion in Social Security benefits in taxable income. In 2002, total Social Security benefits paid were $453.8 billion, resulting in 20.9% of benefits subject to the income tax. Because not all Social Security benefits are taxable, information from tax returns does not reflect all Social Security beneficiaries. In addition, for married couples filing a joint tax return there may be more than one beneficiary. The Congressional Budget Office (CBO) has estimated, using 2000 population and income levels with 2003 provisions of law, that 39% (or 15.6 million) Social Security beneficiaries are impacted by the income taxation of Social Security benefits.

In the 108th Congress, 16 bills were introduced, but did not pass either chamber, that would have liberalized or repealed the taxation of Social Security benefits. This report will be updated as warranted by legislative activity.