Blu-ray Disks with 4 or 8 Layers

Now that the HD-DVD format officially exists in 3 layers, the proponents of Blu-ray are fighting back with technological demonstrations of multi-layered Blu-ray disks.
Hitachi recently showed Blu-ray disks containing 4 and 8 layers for capacities of 100 and 200 GB at an event in Japan.
Although systems for reading 100 GB disks seem almost ready, those destined for 200 GB disks are experiencing further technical difficulties. Indeed, the more layers there are, the more difficulty there is in focusing the laser so that it reads the proper layer without interference from the others.