AA has apparently updated the carry-on policy to explicitly state that, regardless of the dimensions, your carry-on bag has to fit inside of the sizer to be permissible.
http://www.aa.com/i18n/travelInforma...nAllowance.jsp
"The carry-on bag must fit in the sizer and not exceed overall dimensions of 45 inches (length + width + height). The maximum combined linear measurement of the carry-on bag cannot exceed any of the following measurements: 22" long x 14" wide x 9" tall or 114cm (56 x 35 x 23 cm). All carry-on items must fit under the seat or in an overhead bin. "
A little more background can be found on the WSJ blog:
http://blogs.wsj.com/middleseat/2010...age-crackdown/