The only apples-to-apples comparisons you'll ever get are when the two sensors you're curious about came off the same fab line and were engineered at the same time. You can't say anything useful, based purely on sensor resolution, about the latest Canon vs. Nikon.

For what it's worth, the Nikon D800 has a consistent pixel pitch with many of the current-generation cropped-sensor cameras (which clock in anywhere from 12-20 megapixels). As I mentioned above, my Fuji X-Pro 1 modestly outperforms my D700. As such, it's reasonable to expect the D800 to work well, and the DPReview test images support this. Furthermore, unless you're doing wall-sized prints, you're going to be shrinking the D800's output down, which will cover a multitude of sins.