The Reference Guide (Appendix F. Kickstart Installations, Kickstart Commands) states: Note, you can access the network in the %post section; however, name service has not been configured at this point, so only IP addresses will work. Name resolution works fine for me, so you can probably remove this.
I asked a developer working on kickstart and updated the sentence to say the following: If you configured the network with static IP information, including a nameserver, you can access the network and resolve IP addresses in the %post section. If you configured the network for DHCP, the /etc/resolv.conf file has not been completed when the installation executes the %post section. You can access the network, but you can not resolve IP addresses. Thus, if you are using DHCP, you must specify IP addresses in the %post section. It is fixed in CVS and will be correct for the next release. Thank you for your report.