I don't know if this is really a concern just yet. You can only just now pick up 4TB drives, and only in the 6-disc NASes is this an issue. With 6x4TB discs, using two for redundancy, you are just at the 16TB limit. Remember, it is not the disc capacity that is the issue, but the available space.
Mac Pro (2.8GHz 8C, 16GB RAM, 960GB Accelsior, 500GB SSD, 2x750GB RAID0) - MacBook Pro (15", 2.66GHz i7, 8GB RAM, 250GB SSD) - MacBook Pro (15", 2.53GHz, 8GB RAM, 250GB SSD) - MacBook Air (2GHz i7, 8GB RAM, 250GB SSD) - Mac Mini (2.3GHz, 8GB RAM, 500GB) - Pro Pioneer (3x3TB + 3x1TB) - Ultra 2 (2x3TB) - Time Capsule (3TB)