This is a quick and dirty explanation of the process. I'm building as root - which you should probably avoid doing.
Download and install the srpm file from bacula's site (sudo rpm -ivh name-of-srpm.srpm)
You'll need to install the rpmbuild utility:
sudo yum -y install rpm-build
To build the client only:
Install a couple of prerequisite rpms:
sudo yum -y install ncurses-devel libtermcap-devel
rpmbuild --define build_client_only=1 --define build_centos5=1 -bb /usr/src/redhat/SPECS/bacula.spec