My bike has lived outside 100% of the time since I got it and been used to commute in all weather all year round for the last 11 1/2 months. I noticed that when left after some rain or wet weather riding (or after being washed) that the dics would build up a small layer of rust on the braking surface but that was always cleaned off the next time I rode it (5 days a week).

Other than that the rust buildup was not that bad, especially as I never even attempted to clean it off. Just a very small amount in the cooling holes and on the littled link discs that connect the brake disc to the wheel. I'm sure you could clean it off with a wire brush for the holes or somesuch if you're concerted (and making sure they don't get left damp).

At the end of the day remember they are a consumable part. I had my disc replaced last month under warranty (mine had warped) and they look as good as new ... for the moment!