The curious reader might observe that, properly speaking, there is only one plane of focus, so how can a deep volume be in focus ?
The answer is that, in that volume, other effects dominate (like diffraction or lens aberration or film grain or even the imperfections of the human eye), so that for a given depth the appearance of focus is indeed there.

It is also interesting to observe that the 1/3 - 2/3 relationship fails at macro distances (more precisely, at focus distances comparable with the focal lenght of the lens): in those cases, the relationship gets closer to 1/2 and 1/2.
Many lenses have (or used to have) depth of field marks on the barrell, as a quick and dirty help to the photographer: those marks usually embody the 1/3 - 2/3 relationship plus the lens maker's idea of what is "acceptably" sharp.
Notice, finally, that depth of field is a property of a given focal length at a given f-stop: all lenses that are equal in that respect will have the same DOF, regardless of maker and design.
That (plus enlargement) is why, for a similar angle of view (say 45 degrees) you have much less apparent DOF for a larger format: for example, in 4"x5" format, that angle of view corresponds to a 130mm lens, while on 35mm film it is a 50 mm lens - which has more DOF.

See also: DOF preview