Generally you should not go beyond 3 platters; it will get hot, expensive and may be less reliable or durable.
Right now the 3,5" disks are on 500GB-per-platter capacity; so that makes 1.5TB drives the sweet spot. As soon as 666GB (i gotha love that number) platters are out, that means you can create 3-platter 2TB disks; that's where the prices and heat/power consumption of these 2TB disks will start to fall.
So for now, 2TB are bloated disks. The Hitachi even went as far to create a 5-platter (200GB per platter) monster drive just to be the first manufacturer that releases a 2TB disk. Certainly i wouldn't buy it.