The rpm-spec-mode in Fedora’s emacs packages and previous Emacs 23 packages cannot interface correctly with rpmbuild. rpmbuild cannot write to its output Emacs buffer: Redhat Bugzilla #227418. In emacs-23.0.60-0.10.20080330cvs I switched the rpm-spec-mode lisp file to Chip Coldwell’s updated mode based on compilation-mode. This fixes the rpmbuild output buffer at the cost of breaking automatic RPM signatures. Of course rpm --resign or rpm --addsign still signs the produced RPMs.
Update: emacs-22.1-1.fc9 and emacs-23.0.60-0.13.20080424cvs both have this compilation-mode-derived rpm-spec-mode with working GPG signing.