In answer to another question of mine, Matt Grum left a couple links that describe focus shift. One of them linked a very excellent page by Zeiss that explains why they chose a lens design that incurrs significant focus shift at wide apertures for closer focus distances:

C-Sonnar T* 1,5/50 ZM

Information about special features for dealers and users

The C-SONNAR T* 1.5/50 ZM is a very special lens; based on a classical lens design concept from the 1930´s. The additional letter “C†in the name of the lens expresses this designation.

This lens design helps to achieve pictures with a special artistic touch. This lens ‘draws’ your subject in a fine, flattering manner and is therefore ideally suited for portraiture. It renders a sharpness that is slightly rounded, being less aggressive than in contemporary lens designs, but at the same time not soft in its rendition.

Many famous portraits of glamorous and prominent people during the 1930´s used this technique to great effect. These images are characterized by portraying the person in a shining, nearly celestial way. This effect is very well balanced and not exaggerated; therefore many viewers see it in a subconscious way. The trained observer, however, understands the underlining technique and enjoys the results.

This lens design exhibits some additional effects, which should be understood to achieve the maximum benefit from the C-Sonnar T* 1.5/50 ZM:

Because of the above mentioned classical characteristic of the lens the best focus position in the object space can not be kept exactly constant for all f-stop settings.

The passionate photographer might notice a slightly closer best focus in his pictures than expected. When stopping down the lens to f/2.8 or smaller this effect is minimized, so the focus position will be as expected.

In order to balance the performance at full speed and other f-stop settings the lens is adjusted with above described characteristic.

The special features of the C-SONNAR T* 1.5/50 ZM are best used in emotional, artistic, narrative images, portraits or atmospheric landscapes. For documentation or technical subjects CARL ZEISS recommends to stop down the lens at least to f/5.6 or to use the PLANAR T* 2/50 ZM lens.

As I understand it, the effect is due to spherical aberration, or a focus plane that is not entirely flat from the center to the edge of the image circle. This is an intentional design decision that helps create the amazingly soft bokeh of both the Canon 50mm f/1.2 and Zeiss 50mm C-Sonnar f1.5 ZM and similar lenses.

I've seen a few images taken with the Canon 50mm f/1.2 that take this focus shift into account, and they are truly stunning!

http://www.flickr.com/photos/latitudes/2280382988/
http://www.flickr.com/photos/latitudes/2907174824/
http://www.flickr.com/photos/latitudes/2863026063/