When creating an elliptical selection in which one axis of the ellipse is
significantly larger than the other - in other words, a very squat ellipse - the
anti-aliasing of the selection breaks, and the ellipse no longer seems smooth. 
I'm adding an image:
http://www.geocities.com/preacher_mg/gimp.html
the expected result is at the bottom - I achieved it by scaling down a large
ellipse using bicubic interpolation. this is the same result I would expect when
using elliptical selections in other apps like photoshop. What I got however are
the top two, where the antialiasing is somewhat jagged.