Oh fleas are definitely a problem for cats that go outside. If you keep them inside, you can eliminate fleas more easily but it's true that as soon as they go outside, they'll likely pick up fleas.
I'd like to add this to the suggstions gathered here: Diatomaceous Earth. Here is a very informative article about it which includes a source: http://www.richsoil.com/diatomaceous-earth.jsp (The article itself is not selling, just teaching. But the quality of the product is important, you only want to use food grade and the article explains why.)
The drawback with this is that once it gets wet it becomes useless. But it can easily be reapplied. I have used it successfully in the straw I put in the outdoor cat shelters I built. It worked amazingly well. I always like to go for non toxic solutions wherever possible so I was delighted to learn of this.
