[CentOS] post install freezes

Hello,
I'm doing an unattended CentOS 5.3 install in a virtual machine
vmware. I'm redirecting output to a serial console because production
boxes won't have monitors. I'm getting to the point of doing the post
installation then the box freezes. The only command i have in %post is
yum -y update.
I'm not getting no output. Suggestions appreciated.
In the commands section reboot is explicitly called so that at the end
of the install the vm will reboot.
Thanks.
Dave.