Per the New York Times article "Why Can’t the I.R.S. Help Fill in the Blanks?" By Randall Stross, January 23, 2010:
"It’s a stunningly reasonable idea. When you prepare your return, why can’t you first download whatever data the Internal Revenue Service has received about you and, if your return is simple, learn what the I.R.S.’s calculation of your taxes would be?
You’d have the chance to check whether the information was accurate, correct it as needed and add any pertinent details — that you’re newly married, for example, or have a new child — before sending it."
http://www.nytimes.com/2010/01/24/business/24digi.html?_r=1&