I guess the 5D must be some sort of all-around camera. I don't really believe in compromise of high MP and high ISO, I really like Nikon's approach with D3x (high MP) and D3s (high ISO), but for the non-flagship model I guess you need that compromise, and I guess the rumoured 28 megapixel could provide both an upgrade in MP and ISO performance.
In general I think the value of resolution is a bit underestimated, at least for tripod-mounted photography. 400 ppi for book/screen distance viewing distance is a good target, and this is what you can get with high quality printing technology. My personal experience is that there is a clear visual difference between 200 and 400 ppi in printed material. You don't really see individual pixels at 200 ppi, but the lack of micro detail leads to an unnatural pastel-like look, especially highlights are suffering. At 400 ppi at book viewing distance the image looks perfectly natural, and you get the sense that the image has resolution beyond what the eye can perceive, which is what you should strive for. Being satisfied with 200 ppi images is like being satisfied with 8 bit sound.
Also worth noting is that film images have a much more pleasing look if blown up too large than a digital image, so I would say that the resolution requirement is higher for digital than film. There's a charm to film artifacts, but there's no charm in digital ones. The ideas of what resolution is required for a certain size/viewing distance come from the film era, and it certainly needs upgrading to fit the digital era.
You cannot get high ppi count on computer screens yet (they are currently around 100 ppi), but it will come. To fill a 24 inch screen with 300 ppi or both pages of a high quality photo book you'll need around 40 megapixels, and I think that is a reasonable resolution for full-frame, but you will probably have to sacrifice ISO performance then, so every photographer not using a tripod and often shooting at high ISOs would of course want to trade megapixels for better ISO performance.
Of course you could say that high MP count should be reserved for medium format, but I think 40 - 50 megapixels is sort of ideal pixel count for unlimited time into the future (its related to the human eye limitations and normal picture sizes), and since that pixel count is technically possible with good quality for 35mm I think we should eventually get there. At least with some camera body... entry level full-frame might not be the right one.