There has been some sort of update to "libpaper-1.1.23+nmu1": http://packages.debian.org/changelogs/pool/main/libp/libpaper/libpaper_1.1.23+nmu1/changelog.html all changes seem to be in libpaper-1.1.23+nmu1/debian or subfolders. So probably they do not concern any of the files used by Gentoo. Therefore maybe it makes sense to use libpaper-1.1.23+nmu1 as source archive, but without version bump. Then there would be a valid SRC_URI again without triggering unnecessary rebuilds, solving bug #269356 (that has been marked invalid by the reporter). (the SRC_URI in libpaper-1.1.21 is still valid)
+ 08 Mar 2010; Sebastian Pipping <sping@gentoo.org> libpaper-1.1.23.ebuild: + Fix SRC_URI (bug #292706) + Thanks for reporting!