[pic of MSI SLi MXM test platform]
doesn't it look cool
Actually that's not like what you're talking about, it's just smaller cards on a bigger card, kinda the opposite and worse than what you were suggesting. Not only do you still buy a card (a small and very expensive one) but you also have form factor and compatability issues.
The biggest barriers to replaceable VPUs is their size, power draw and heat; as for the replaceable memory, even if they did put GDDR4 on a stick, the pins required for 256bit memory would be incredibly tightly packed or increadibly large, either way complexe, expensive and more fragile. Each stage of the setup would add $$s to the equation, and once the marketing/accounting department got their minds wrapped around the realization that nothing has really changed, the people willing to pay $500 for a new card will also pay $500 for a new chip, then what you have now is still a $500 upgrade, only the money goes directly to ATi and nV. And for that reason alone, it's unlikely to happen, because if it were at all possible when the VPUs were simpler both ATi and nV would've done it for the sheer profitability, however, that it doesn't exist even as a working prototype shows that pretty much it's more difficult to implement than it's worth.
The reason the MSI solution is great is because it's for laptops. And if they bother to sell them as SLi expandable/compatible, instead of starting with both slots populated, you would have a good upgrade path in the laptop market to balance power consumption and performance as well as lengthen the life of your laptop. Of course you'd end up with the same choice as in desktop, two GFGO7800s in SLi or one GFGO7900GTX? I'd take the single slot solution going from a GF7800 to GF7900 instead of doubling up, especially in a laptop.