My opinion is that a 6 megapixel sensor is needed for most of us who only print at 8x10 from an inkjet or display on a 23 inch monitor. PAD displays at 800x531 pixels, which is less than half a megapixel! Even an HDTV monitor can only display about 2 megapixels of image. Thus, a siz megapixel image has to be downsized to display on PAD or on a 55inch HDTV. Nikon produced 6 megapixel sensors back in the days of the "old, outdated" D40 and D70. Cameras of that era, 2005-2006, are more than adequate for PAD or 8x10 prints or display on computer monitors or HDTVs. Everything we view on computer monitors which was shot at 6 megapixel is being downsized in the background anyway.
For those who print larger, say 11x14, 12 megapixels are needed. For those who are going to print poster size, 20x30 or 24x36, 24 megapixels are needed. I have recently printed a 24x36 poster from about one third of a 36mp D800 image.
So "need" depends upon intended use. Many of us need 12 megapixels, a few of us need 24 megapixels (the new D600 and likely the soon to appear D400), and a very few of us need 36 megapixels.
How high will the FX size sensor go? Maybe to about 50 megapixels in a D4x. It is said the 36 megapixel D800 sensor starts to show reduction in image quality at around f8 which is actually the sharpest f stop on many lenses. The D600 at 24 megapixels should not start to show degrading at f8 and perhaps not at f11 making it a "better" choice for FX. At 50 megapixels the FX sensor may start to be diffraction limited at f5.6 which is the largest f stop available on some modern zoom lenses at some focal lengths. Thus, a 50mp FX sensor may need to be shot at f4 for maximum resolution, making it a tool of limited use.
If we go to larger lens coverage such as provided by medium format we can have larger megapixel sensors before we reach diffraction limitations.
So can anyone produce digital sensors which can be used with 4x5 or 8x10 view cameras? Then you wouldn't need a whole new camera system, just a retrofit, and you can have very large digital sensors. To my knowledge no one has been doing such a thing.