I've always heard the warning about leaving a gun in a leather holster. I wonder though, if I carry the gun around in the holster for 16 hours a day without any problems, why is it suddenly going to rust if I leave it in the holster for that extra 8 hours?
My Glock stays in the holster pretty much all the time. The inside of my Don Hume holster is treated with leather dressing, and the outside with Renaissance wax. I think this, combined with wearing a undershirt between the holster and my body, keep any sweat from soaking into the leather, possibly minimizing the corrosion risk. Of course, plastic frames can't rust, and the Tenifer metal finish is pretty durable as well.
I would think traditional blued guns (especially if the holster gets wet) would be most susceptible.