Depth of field is a function of the focal length and aperture of the lens we are using and the distance to the subject we are focused on. This is determined by the laws of optics and the physiology of human perception.
However, in a similar way to how high dynamic range imaging can extend an image sensitivity, we can extend our depth of field by stacking a number of images shot while focused at different differences and then using smart software to combine these so that we have an extended depth of field.
Below are the articles we have currently on this topic:
Photoshop CS4 and Extended Depth of Field