Why is this anti-aliasing artefact happening? Shown below is a render of a model of mine.



Larger version here

As you can see in the circled portions, the anti-aliasing is pretty bad. How do I improve it?

I've tried increasing the samples per pixel and the sample width, but neither seem to help it. Is there any other way to correct it?
Looks like you have transparency on the Island material, switching it off seems to do the trick,