Rust doesn't form in any non-negligible amount if the concrete remains uncracked. As long as the foundations don't crack, it's not a problem. And if cracks are found quickly enough, they can be sealed.
Shavethewhales, I have to disagree with you on the use of epoxy-coated rebar. Most of the foundations I've seen (which, incidentally, are in the amusement industry) don't have any sort of protection on the rebar, and that's how I remember learning it. Protective coatings on rebar are generally used where there are other factors that may make rusting rebar worse, such as road salts, etc. Epoxy reduces the bonding between rebar and concrete, so it's not ideal to use.
Edit: It's worth noting that uncracked concrete provides sufficient rust protection because the alkalinity of concrete helps to maintain a passive protective layer around the rebar. Cracked concrete loses that protection (and also allows water intrusion).