Looks like we have an answer.
"The problem is, as NASA's Douglas Ming (the Merciless) has pointed out in a recent paper, the experiments to search for carbon involve heating soil samples to a few hundred degrees and sniffing the vapours. If the soil contains carbon molecules and perchlorates, the carbon molecules will simply burn up. No wonder they couldn’t find any."
Fred
http://timesonline.typepad.com/science/2009/06/is-there-a-mars-conspiracy.html