its not just fish eye lens. it happens with a lot of lenses, especially when they are wide open. there are ways to cause it or to avoid it. what are you trying to do? still photos i take it?
BTW, thats not a good example picture really.
here is a decent simple explanation of the technicalities of things:
http://www.vanwalree.com/optics/vignetting.html
grant
notice the black fade coming from the corners in this random picture i took. you can see it the most in the floating arm and in how dark the discoball is.
I cranked it up higher in the following two pictures just as an example:
not the best examples, but it should give you can idea