Apple released version 2.1 of Mountain Lion's Server app today. The most notable change is that you can now configure DHCP from the Server app. However, if you implemented custom DHCP due to the lack of this feature in the previous version, you will have to undo your work. As per a previous post, the file bootps.plist in /etc had to be configured to get DHCP functionality. The new Server app v2.1 also use this file and as expected, if you have a custom one, it tends to break Server app's DHCP function. You'll notice you can't configure DHCP in Server app or enable it.
To resolve this issue, simply move /etc/bootps.plist. Server app will re-create this file soon as you start configuring DHCP.
mv /etc/bootps.plist /etc/bootps.plist.custom
It does mean you'll have to re-create your DHCP scope in Server app. Fortunately this should be real quick given Server app's ease of use!
If you had any static IPs configured in /etc/bootptab, you might want to re-create them in Server app so you can see them! For consistency sake move your old bootptab file too.
mv /etc/bootptab /etc/bootptab.custom
Finally setting up DHCP in OS X Mountain Lion Server is how it should be, configured via the Server app!