I would love this!
However, like many people have said, I cant see this becoming a reality.
Also I think someone else has mentioned before; what I'd really like is an updated 17" MBP in the form factor we last saw it in rather than the "you cant do anything to upgrade it" rMBP form.
I got my early 2011 17" MBP brand new with upgrades to a 120GB SSD and an anti glare screen.
Since then its had an SD card reader or USB 3 in the express card slot. a move to 8GB of 1600mhz RAM, A 240GB SSD and a 1TB HDD in the optibay.
I've got the 2012 BT card on its way so I'll be able to use handoff etc and RAM prices are getting to a point now where it wont be too long before I go to 16GB.
This ability to upgrade components has ment that I really haven't needed to think about upgrading to a completely new machine and should hopefully still get another couple of years from it.
For me this its a real plus that every so often I can spend a relatively small amount and get that "ooh feels like new again" feeling. But Apple want people to buy from them and refresh their machines more often, not less. So as we're seeing they are even moving away from being able to easily upgrade ram. Hence my dream machine will never become a reality