This feature is new to Photoshop CS4, although stacking was available in the Photoshop CS3 Extended version. I believe that the Extended versions give you more control over the stacking algorithm but I haven't seen it in person. There is some ambiguity as to what this technique is called. For many years astro-photographers have used a technique called "stacking" which requires a number of images of the same subject that are all aligned and then a mathematical operation is performed on them such as 'add' or 'average' which results in a sharper, less noisy image. This is different than "stitching" where multiple images are aligned side by side as opposed to directly on top of each other. "Stitching" is used to create panorama images. Adobe, at one place in their online help for Photoshop CS4, refers to the stacking technique as "auto-blend focus". However, the controls don't mention "focus" anywhere. There is a radio button selection of "panorama" or "stack images". Some online resources have referred to this as "focus stacking" or "depth-of-field stacking".
(Keywords: DoF stacking, depth of field stacking, focus stacking, auto-blend focus, auto-blend stack, Photoshop, CS4)