/usr/include/rpm/ugid.h, which comes from RPM's rpmio directory, is missing from rpm-devel-4.0-4.i386.rpm. This is a problem when code #includes <rpmbuild.h>, which #includes <misc.h>, which tries to #include "ugid.h". This causes a compilation error when trying to use rpm-devel to build another package.
Fixed in CVS, will be in rpm-4.0.1 and rpm-4.1.