I think that the display-technology has to move ahead a little first, before we see 6, 8, 10, 12, 18 MP video-cams.
Pany AF100 reportedly has a 12MP sensor.
Imagine the dynamic range and high-ISO handling of an APS-C or even FF sensor with 6 MP, using modern technology?
Sony PMW-F3 reportedly has 4X size pixels.
Then you could have a full output for stills, and if the video output has downsampling applied real time at capture you retain optimal usable detail at practical data rates for both applications.
Problem is that, under those conditions, video could have moire and aliasing. Which is why the AF100 has an OLPF optimized for video and does not capture full 12MP res stills.
And the higher pixel count also allows for video "pixel zoom" through crop without blocking due to digital scaling.
Yup, GH2 does this.
18MP at 60fps would be shifting about a gigapixels a second. I'm not sure how many instructions per pixel it takes to de-bayer and pixel bin, but I'd say that is probably within the realms of current processing technology. Maybe not necessarily at consumer levels.
Yup, Red MX does 5k @ 120fps.