The effect is called vignetting and its a by product of inferior lens design. For most of photographic history photographers have attempted to remove vignetting from images. This changed most notably, however, when photographers started using the Russian Lomo brand camera because of its unique and very strong vignette. The effect can add a layer of visual sophistication to an image, especially when viewed on screen.
I achieve this effect by selecting and area around the border of the image about where I want the vignette to become noticeable using the lasso tool.
Next right-click in the center of the selection and choose 'feather'. Select a radius of between 75 and 250 depending on the overall resolution of the image. I typically select 150 for my 6MP D50 images.
Right-click [again] in the center of the image and select 'select inverse'
Right click on the border area which is now selected and select 'layer via copy'
On the layers pallet select the new layer and change the blending method to 'multiply'
Finally, fade the opacity of the layer using the slider in the layers pallet to make the effect look natural.
CTRL+ALT+E to merge all visible layers
Viola