Not really surprising: even in a well sealed system, if you're not actively changing and filtering the air in the cabinet, you're gonna get dust settling.
I've been installing "home furniture" wall shelving in combination with london style brackets from Masters. The main attraction is that its the most economical 30cm deep shelving I could find which is great for placing LEGO modular buildings (and large transformers ). The london brackets aren't the most elegant looking thing in the universe but they are the strongest and least obstructive bracket available for a very reasonable price.