I have two Kimbers, albeit both compact models, and would not part with them for anything... My Compact Combat Carry is a matte stainless slide on a black lightweight frame. However, my all steel Compact Custom is roughly analogous to the Custom you are considering, and my experiences should apply.

I've had no problems with rust with the finish (black oxide, actually) on my gun. (Others - one person in particular - have reported quite the opposite with their finish, but from what I hear, I believe my experience is more typical.)

To put it in context, my gun was used for daily IWB carry for about a year and a half. This included one wet Seattle winter - 9 months. <g> - and all the usual "body contact" from IWB carry. Now I have switched to carrying the Combat Carry. More for it's weight and features (sights, safety, etc.) than for it's finish. I should also point out that I am a believer in cleaning and oiling. Since it' was my carry gun, I kept it clean. An average of twice a month I would clean it to insure function. (Strip it, remove lint, etc. , clean and re-oil, reassemble.) As a final step, I'd rub the entire exterior with a lightly oiled rag. This may have contributed to no rust, but to me it's just *normal* maintenance.