This can be achieved using Photoshop pretty easily too, though with more steps for the action. It comes in handy if you wanted to do it to a batch of photos without having to leave Camera RAW and adding another step to the workflow.

Example:
http://www.flickr.com/photos/21437581@N06/4135941084/sizes/o/

Not having the edge 100% sharp can add to the effect of it being an ‘in-camera’ effect, not a digital post effect.

http://laci.blog.hu/2009/11/08/emlek_69

http://graphicssoft.about.com/od/gimp/ss/vignette.htm

Vignetting of RAW images in CS4′s Camera RAW editor offers two options: 1) “Lens Vignetting” of the original, uncropped image and, 2) “Post Crop Vignetting” of images which have been cropped. You can use either option on uncropped photos (though I think the “Lens Vignetting” option yields superior, more natural results for uncropped images); for cropped photos, you need to use the “Post Crop Vignetting” else you will lose the vignetting effects at corners or edges which were lost due to cropping. Using the “Filter–>Distort–>Lens Correction” filter in CS4 allows for vignetting cropped images but does not offer all the options one gets when using Camera RAW, specifically the “Roundness” and “Feathering” options.