Dann: Go to the attribute editor for the camera you're rendering from and choose the environment tab. This will allow you to change the background for you renders.

UG: I'm not a big fan of feathering for the most part because it washes out (blurs) the objects edges (something I definitely don't want). I'm more concerned with figuring out where the black line is coming from so I can remove it properly.

Interesting about the Pre-multiply, Dann.