I'm surprised it took this long to return to multi-GPU cards.
It's not like we haven't been there before. Just look at 3dfx and ATi in the past.
CPU's stopped building faster single cores and went to multiples, but Intel are doing the same as both Nvidia AND ATi, they are putting multiple dies on one chip and hooking them together to increase the number of cores, and slowly making bigger single dies over time, while AMD are making single larger dies instead of sticking 2 smaller ones together.
Graphics people are just heading the way of Intel and putting smaller chips together on the same card, a bit like Intel. More cost efficient in some ways, even if the end user doesn't like it so much.
Faster single GPU's will come eventually, but there isn't really a huge rush or need for them right now when they can just put 2 together.