Some researchers have given strong evidence that sitting up straight is not only unnatural, but could cause you back problems down the line. They presented their findings at the RSNA Conference yesterday in a talk titled The Way You Sit Will Never Be the Same! Alterations of Lumbosacral Curvature and Intervertebral Disc Morphology in Normal Subjects in Variable Sitting Positions Using Whole-body Positional MRI.
A quick summary can be found here. Or, if you prefer a news article... try this link instead.
Anyway, kick back and live better.