Image sensors themselves have depth, like infinitesimally small buckets that collect photons instead of water, but on top of these buckets is a perfectly flat and annoyingly reflective surface.
"Because the surface of film was never completely flat, compromises could be made," says John Carlson, Product Manager for Pentax. "With a completely flat sensor, lenses need special anti-reflective coatings to minimize ghosting and flare, and they also need designs that correct for spherical aberration. This is done by using aspherical lens elements and by designing lenses that minimize the field curvature, basically, flatter final lens elements."
With the exception of Sigma's Foveon sensor, sensors don't have separate layers for recording Red, Green and Blue, as does film. Light from all three colors must be aligned properly, on the same focal plane, or you'll have problems with chromatic aberrations like color fringing.
And since the original design standard for many SLR-turned-D-SLR lenses is 35mm film, the image circle they create is intended to cover the area of a 35mm frame: 24x36mm. With the Nikon D3 or the Canon EOS-1Ds camera line, there's no need to apply a magnification factor to focal length because their sensors are the same size as 35mm film.
However, if the sensor is smaller than a full-frame, and a majority of them are, a portion of the frame becomes cropped because the image circle is much larger than the sensor actually needs. If the sensor is two-thirds the size of a full-frame, a 28mm focal length will look like a 42mm focal length, as if you had zoomed into the scene you're photographing.
To maintain the same angle of view, which is especially important with wide-angles, manufacturers have to shorten the focal length. That's why you often see a 35mm-equivalent focal range listed with interchangeable lenses. Because so many of us still think of focal range in terms of the 35mm standard, manufacturers include that as a reference.
That way we know a 12-24mm DX Nikkor, designed for Nikon's DX sensors (15.8x23.6mm) will have a 27-52.5mm perspective, or a Canon 17-55mm EF-S, designed for its smaller APS-C-sized sensors (15x22.5mm), will perform like a 27-88mm EF lens, and so on.