The ingredient is considered safe by regulators (and Tom's, of course). Some people choose to limit their exposure to it. SLS is a foaming agent used in many body care products. I try to avoid it and buy shampoos and things without it.
Well, I recently saw that Tom's has finally released a line of toothpaste that is free of SLS. They have a new "patent pending" ingredient in place of the SLS. I don't know anything about it, other than it is called Glycyrrhizin and is a sweet-tasting compound purified from the root of the licorice plant.
While I still use the Jason Natural Cosmetics Sea Fresh Plus Coq-10 Toothpaste, I have been buying Tom's for my kids since they have some nice kid-friendly flavors (the strawberry and the mango are popular in our house). Next time I am at the store I might try a new tube of the SLS paste. There are some pretty interesting flavors available in the new line . . . fennel anyone?