Predators are definitely a problem in many parts of the country. I know of everything from pet mud turtles that have been carried off and eaten by birds to 12 inch adult elongated tortoises being eaten by groups of raccoons.
There is an excellent article by Darrell Senneke of the World Chelonian Trust available in the hundreds upon hundreds of WCT pages on turtles and tortoises. It is entitled "A secure habitat" and can be found via the search engine on the front page of the site or via the following cut and paste link:
http://www.chelonia.org/Articles/secure.htm
It covers a wide variety of predators as well as solutions.
There are also a number of new tortoise articles available on the site. Just click on the link below or go to www.chelonia.org and look under "new articles" for pictures and info on sexing flat tailed tortoises, differentiating the Burmese mountain tortoise subspecies, as well as a number of care articles, both originals and translations, written and translated into Spanish.
Doc

World Chelonian Trust