It's called vignetting.
From wikipedia
In photography and optics, vignetting is a reduction of an image's brightness or saturation at the periphery compared to the image center.
Although vignetting is normally unintended and undesired, it is sometimes purposely introduced for creative effect, such as to draw attention to the center of the frame. A photographer may deliberately choose a lens which is known to produce vignetting. It can also be produced with the use of special filters or post-processing procedures.
From a purists perspective it is considered a bad thing, often the product of the image a lens produces not being big enough to cover the whole sensor (or film) in an SLR.
The interesting thing is that although this effect is often seen as a poor quality of a lens, some people like the effect and use it artistically.
I think it can be used well, but it is far too often over used. But it doesnt necisarily make it a bad picture, its just personal taste.