If you are running a wireless network, you risk someone being given an IP if they manage to get through your WEP and connect. If you are on a wired LAN, you risk someone being able to walk into the building with a laptop and plug into a network port and have internet access. They wouldn't have access to servers or clients without getting past other domain security features.
Since all DHCP does is give someone an IP on your network and some basic information like what the default gateway is and where your DNS server is, the security risk is exactly the same as the risk in providing someone with those things after they already had basic connectivity to your network.
In my opinion, it's not a big security problem. There are much more important areas of security to focus on.
ShackDaddy