From Wikipedia, http://en.wikipedia.org/wiki/Depth_of_field:
“Same picture” for both formatsFor the common “same picture” comparison, i.e., the same camera position and angle of view, DOF is, to a first approximation, inversely proportional to format size (Stroebel 1976, 139). More precisely, if photographs with the same final-image size are taken in two different camera formats at the same subject distance with the same angle of view and f-number, the DOF is, to a first approximation, inversely proportional to the format size. Though commonly used when comparing formats, the approximation is valid only when the subject distance is large in comparison with the focal length of the larger format and small in comparison with the hyperfocal distance of the smaller format.
"To maintain the same angle of view, the lens focal lengths must be in proportion to the format sizes. Assuming, for purposes of comparison, that the 4×5 format is four times the size of 35 mm format, if a 4×5 camera used a 300 mm lens, a 35 mm camera would need a 75 mm lens for the same field of view. For the same f-number, the image made with the 35 mm camera would have four times the DOF of the image made with the 4×5 camera."