It is irresponsible to brand anything a carcinogen without providing the source of information. If SLS is carcinogenic, there should be hard evidence somewhere, but a search of the medical literature failed to turn up any reports of adverse effects from SLS. I did find that, like soap, it can sting if it gets into your eyes and that, at higher concentrations than you'll find in any shampoo, SLS can irritate the skin.

Compare this to similarly alarming reports about diethanolamine (DEA), another shampoo component. Here, a study from the National Toxicology Program linked residual levels of DEA to cancer in laboratory animals. Although the study didn't even address the question of whether DEA can cause cancer in humans, the FDA announced that it would evaluate the data to determine the risk, if any, to consumers. I could find no record that the NTP or the FDA is looking into SLS and no report of a link between SLS and human or animal illness.

SLS has a long history of use in shampoos and other personal care products and overall I don't think you need to worry about it. If you'd rather not take any chances, you should have no trouble finding alternative products that do not contain it.