Get a gummi stone and try to remove the rust yourself, first. A gummi will remove a lot/all rust without affecting your base/side bevels.
If it still doesn't come off satisfactorily or if the edges are pitted, get a stonegrind as Jer suggests.
If you keep your edges tuned and polished and do as Richie-Rich suggests, rust shouldn't be a problem in the future. Stonegrinding leaves striations in the edges which can quickly cause rusting (within hours sometimes on the drive/flight home). Polish out the striations and the problem usually doesn't reoccur. In the future, attend to any dings in the edges which also promote rusting and can certainly effect your skis' performance on hard snow.
Or you can just sharpen you side edges and forget about the rust. Superficial rust can be skied off. However, if ignored, the edges will eventually become pitted which will be noticeable even to a lot of non-racers.