Since you already have dev-java/{xerces,xalan} packages, it only makes sense to have one for fop too. Xerces is the XML parser, Xalan is the XSLT processor, and FOP is the formatting objects processor. That is, from an XSL formatting object, it's FOP's job to make it into something useful like a PDF.
Created attachment 4639 [details] ChangeLog
Created attachment 4640 [details] fop-0.20.4.ebuild
is there any reason this package is not built from source?
Sorry, not really sure. :) But um... it's a bunch of .class files anyway (Java bytecode). So I don't think it would make much, if any, difference if it was compiled from source anyway (other than take more time).
Already in portage as dev-java/fop-bin same version and everything, if it is requested, I will write a fop-src ebuild.