I've just commited a working PSP ELF packer to subversion (under svn://svn.pspdev.org/psp/psppacker), it is based in part on the version John _K did a while back but improved somewhat, plus it has a nice simple PC tool to pack you executables. It also works with both normal ELFs (even pre-built applications) and PRX files.
For all the lazy windows 'tards out there I have cross compiled a version for windows using mingw, it is available in the www.pspdev.org psp tools section.
Enjoy and post any feedback/bugs etc. here.
Oh and you need to update your pspsdk, again, to build it ;P