The sensor actually uses 25.5 million pixels, but due to the unique layout of the sensor Sigma claims it can record twice the detail of standard models.
Soon as they say "50MP, but actually 25 ... " you know it's a Foveon. So the resolution will be ... 25. But the color will be spectacular. I admire their bravery in breaking the layout mold a bit with this one. I'd sure like to try it in my hand, though (larger hands than standard).