Some of the steps from my test of cobbler / centos / virtualbox.

1. Download Centos 6 minimal ISO, install it on a virtualbox guest #1.
   1. Call it centos6a.
   2. Configure it with ~8GB root partition, 1GB memory & 2 network interfaces, eth0 bridged adaptor, eth1 internal network.
   3. Unless you really want it I recommend disabling SELinux.
   4. Manually configure eth1 on a private network (system-config-network), you will configure dnsmasq/cobbler later to use this private network
2. Install & configure cobbler & cobbler-web as per this HOWTO.
   1. Review settings & modules.conf in /etc/cobbler/ – particularly dhcp & tftp settings.
3. Copy the centos 6 iso over to centos6a and mount it
   1. mount -o loop /tmp/c6a.iso /mnt
4. Import the centos 6 iso
   1. cobbler import –path=/mnt –name=Centos6 –arch=x86_64
5. cp /etc/cobbler/dnsmasq.template /etc/cobbler/dnsmasq.conf clean up & restart dnsmasq
6. cobbler sync
7. Create virtualbox client #2 (centos6b)
   1. 512MB memory, 1 network interface (eth0 internal, adaptor type PC-Net III (for PXE boot)), ~4GB diskspace
   2. Settings -> Motherboard, move network to top of boot order
8. Boot centos6b – it should install from cobbler

Other resources: Start Here