According to Red Hat 6.1 Reference in Chapter 10 Kickstart Installations, the DHCP next-server option should select the NFS server from which the kickstart file comes. This does not work. The client always tries the DHCP server, not the server specified on next-server. This appears to be a pump implementation error.
We've been using the next-server for this purposeas well, and have also noticed that it broke in 6.1, so we had to move all of our kickstart boot files over to the DHCP server. Hopefully this will be fixed soon...
Fixed in the piglet beta.