I was browsing Costco's website when I came across this mini-dehumidifier (Royal Sovereign Silver Mini Dehumidifier $29.99 Item # 908759) that seems like it would be perfect for camera cabinets. I'm now living by the ocean and the humidity is quite high. I'm going to give this a go and report on how it works. See my previous blog entry on keeping cameras and lenses fungus free.
Update: Unfortunately, Costco.com was sold out of these by the time I got around to ordering one. If anyone discovers them at another store for around the same price, please let me know.