This almost always is a simple address or netmask problem. Make sure you are using the latest dhcpd (http://www.isc.org/products/DHCP/dhcp-v3.html) conf syntax and then you can diagnose what is going on by opening up another terminal and running arp -a during the boot... there is a good MAC-whacking script at http://www.slac.stanford.edu/~alfw/PXE-Kickstart/ if you want to automate the process.
...and stay tuned, we are striving to update the debian-diskless HOWTO.
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