Roel,
A lot of the guys posting messages here have keyboards costing $4000 or more. But that is not an avarage keyboard. I'm talking about keyboards in the range from $1000 thru $1500. If you look at the specifications of such a keyboard you really have the specifications of a 20 year old PC.
When I'm in a music-shops I'm always amazed that the revolution that has taken place in the PC world is not happening in the keyboard world.
What I described is a Media Station as you described it, but in the shape of an arranger keyboard (with 61 normal size keys). When there will be a machine in the shops for $1200, that you can use as an arranger keyboard with a 600*800 color display, a hard drive, unlimited memory, that can be loaded with any sequencing and arranging software and softsynths, and can also be used as a normal PC, then people will buy it! They could easily sell a million of those! And they can sell it for such a low price because it uses standard PC parts (except for the black and white keys).
And there will be heavier models for (semi)professionals too, equiped with high quality soundcard synths.
So I understand that you won't spend the night in a sleeping bag before the music-shop to get the first one when it arrives. But it will arrive!

De mazzel!
Jos