Doug,

Consumer grade acids, such as muriatic that can be purchased at a pool supply store, can resolve most rust stains. Simply pour a small amount on the rust, allow it to dwell for just a moment, & then scrub with a stiff brush. Be sure to rinse the acid with a neutral cleaning product, as the acid will continue to abrade the grout if not removed thoroughly.

Note that some porcelains can be delicate and may be etched or damaged by the acid, so test in an inconspicuous place first. It can also be a good idea to water down the acid, first.

A less toxic option that works well on the tiles, but no so well on the grout, is a pumice stick such as this: http://www.walmart.com/ip/United-States-Pumice-HDW-12-Heavy-Duty-Scouring-Stick/23098372