Why do the new iMacs have mobile video cards?
Four factors are in play for choosing how to design a system:


Space
Cost
Performance
Reliability


I certainly won't speak for Apple's engineers, but buying in volume lets a video chip manufacturer pack more performance in a lesser cost package. Any performance that isn't balanced with the rest of the system is wasted performance. One would presume this was the optimal choice for the new iMacs given the state of components available.

Performance is also about efficiency - so if the raw benchmark is big enough to drive three huge monitors on the 27 inch model, why not choose a smaller mobile chipset, manage less drivers in the OS, reuse the same chip on the mobile line and free sourcing, engineering, testing and repair inventory assets for other more pressing problems to solve?

We will soon have benchmarks to see whether the designers did their jobs well this revision. My limited testing shows the new iMacs to scream compared to old MacPro and iMacs which were no slouch in the graphics department by my standards.