1) PIE follows the current CSS3 spec in that it requires the 'fill' keyword to be present in order for the center area to be filled in. (Other browsers don't yet follow this piece of the spec but I expect they soon will.)
2) There is a bug in PIE where the solid border is still rendered when border-image is present, when it should become hidden. This is being tracked as http://github.com/lojjic/PIE/issues#issue/51
and I already have a partial fix.
BTW, your testcase document is awesome! I know you're still adding stuff to it, but would you mind at some point submitting it to the set of tests in the PIE codebase?