James' link below is a good, though somewhat over technical explanation. However, remember it has nothing to do with film vs digital, its about the size of the image recording area - i.e., the chip (or sensor) in digital, or film in a film camera. While the 10D and most digital SLRs have 1.5x or 1.6x magnification factors because of their smaller-than-full-size chips, cameras like the Canon 1DsM2, the 5D, and the Kodak 14n/14c, which have full-size chips, have exactly the same DOF as 35mm film cameras, using the same lenses, distance, f-stop, etc.
Here's another explanation, maybe simpler. We all know DOF is dependent on focal lenght of the lens, f-stop, and focusing distance. If you use a 300mm lens on a 35mm camera (full size chip) you get the same field of view (the framing of the subject in the viewfinder) as someone using a 200mm lens on a APS (1.5 or 1.6) chip camera, like the 10D, 20D, any Nikon or Fuji, etc. BUT..... because you're using a 200mm lens instead of the 300mm lens, you have more DOF, partly because you have to be 50% farther from the subject in order to get the same framing. You can extrapolate that to any lens/distance/format/f-stop combination. And by the way, this is why larger format cameras - 6x7, 4x5, 8x10 - have even shallower DOF and require more light (hence a smaller f-stop) to achieve the same working DOF. Hope that makes sense.
Andy Pearlman Studio