Works fine for me on gentoo, albeit not using the ebuilds.
Code:
ebook-meta /home/kovid/pdf/Extract_ANAT_Annual\ Report_2008.pdf --to-opf /t/t.opf
Title : Extract ANAT Annual Report 2008
Author(s) : Unknown
Book Producer : Adobe InDesign CS3 (5.0.4)
OPF created in /t/t.opf
You likely have a problem with your python install. Try re-emerging python.