Chased down an odd behaviour between ISC Dhcp (3.0.4 in my case) and some Mac OS X clients. You'd see the clients doing a DHCPDISCOVER, receive the DHCPOFFER from the server but no DHCPACK would ever show up in the logs. After some sniffing and some trial and error, we found the culprit:
After changing it to:
the Mac OS X clients started working again.