Just curious... where are you keeping your computer that causes it to rust? Every case I've had so far - all steel cases - has never rusted. The components inside should wear out long before the case rusts in normal conditions.
If you have it in a very moist or caustic environment, you're most likely going to ruin the innards first anyway.
With that being said, the Thermaltake Kandalf LCS is an excellent case. It's HUGE and HEAVY, and primarily steel. It's mainly pricey due to the liquid cooling (which is mediocre), but the case itself is definately worth the cash. Having places for tons of hard drives, large expansion cards (8800GTX fits without a problem), huge power supplies, pre-drilled water tubing holes, radiator mounts, and pump / reservoir mounts are all pros.
On the other hand, the Thermaltake Tai-Chi, and many of the high-end Lian Li cases are very popular. I haven't tried them myself, but many people swear by them.