this is too simple, by using photoshot feather brush , or u can select the area and set the selection feather then hit delete , simple
It could be the thing called vignetting. Look more here: http://library.creativecow.net/articles/zijlstra_sytse/creating_a_vignette.php
Pretty simple way is to just get a soft black brush in photoshop and drag it vertically with a slight curve on each side.