I too have the same problem. Got the below error in installation logs.
"zimbra-core-8.0.4_GA_5737.RHEL6_64-20130524120036.x86_64.rpm is not signed".
and my os details are
# cat /etc/redhat-release
Red Hat Enterprise Linux Server release 6.0 (Santiago)
# uname -a
Linux zimbra.satya.com 2.6.32-71.el6.x86_64 #1 SMP Wed Sep 1 01:33:01 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux.
I have installed zimbra7 version. But, i didn't get any problem like this.
Please help me to install Zimbra8.0.4.
Thank you in advance.