At some point an enterprising company is going to capitalize on the market for a functional DVD file cabinet (if they already haven't). Portability of 400-600 discs is rarely if ever necessary. After all, why the hell would you need to take 500 DVD's on the road?
At nearly 600 DVD's, I need functional storage not portability.
As DVD burners and cheap media become more and more prevalent the market is ripe for a well thought out solution for storage that is above and beyond a super high capacity disc wallet. My solution would be a modular file cabinet setup complete with hanging pages and a tabular system. As it is, my disc wallets stacked up are nearly 2 feet tall. I think my collection has finally outgrown wallet type storage, it's time to move up to the next level.