I believe that we will see these introduced in the near future. I don't believe it is a sensible idea to have 4 PCI-Express connectors, sometimes requiring 2 PCI-Express power connectors, built in to the motherboard. I believe that we will see the introduction of a multi-GPU capable external graphics card enclosure suitable for dual-slot graphics cards. I believe that this will be a separate powered unit or should require a special single power connector from the PSU.

For example: ATX motherboards simply are not big enough for 4 x 8800GTX's or 4 x X2800XTX's. We already have multi-HDD capable external hard drive enclosures using separate power, and Asus have already created a device that will allow 2 x PCI-Express graphics cards / a graphics card and a physics card, but it's aimed at laptop users. I'm sure that we will see this introduced on the desktop very soon..

Hmm, that would be quite interesting, though inconvenient for many users. We could see a change in motherboard and case designs, but that would be quite a big, confusing change.

Following up on the point about the G80 and R600 GPU's being the largest we will ever see... we could have 4 x single slot graphics cards... No doubt a single slot 8 series would trounce a dual slot 7 series in most/all benchmarks. As technology advances and gets smaller, maybe we could see single slot graphics cards being top performers?

This will be interesting! I want to get quad SLi!

This is freaky...
This article was published yesterday: http://www.theinquirer.net/default.aspx?article=37607

Hmm, external PCI-E, nice! Will be very intersting... Laptops are indeed the biggest use for this.

Hmm, this could make things interesting... A low power laptop that can dock to become a high end desktop would be interesting and very useful!