Ok so here is the deal this ticked me off as well. There are 2 things with this.
In your ImageButton Selector, you seemed to copy the attributes for the right corners twice in each corner tag.
The second is a bug in android up until version 3.0. When specifying the corners separately, the bottom left and right corners get flipped.
http://code.google.com/p/android/issues/detail?id=9161
I have extracted the values out to dimens and put them in two different files,
res/values/corners.xml – with the reversed stuff
res/values-v12/corners.xml – with the sane values in them.