Tobacco Hornworm (Carolina Sphinx Moth)

Nice find! I believe this may actually be a Carolina Sphinx Moth caterpillar. The markings are more consistent with that species. Please take a look at the link I provided for comparison. Here is another link to compare to Manduca quinquemaculata: http://bugguide.net/node/view/65586
Notice how the markings on the side of M. quinquemaculata are V shaped,unlike your caterpillar which has slash shaped markings.