What type of DHCP server are you use (Microsoft, ISC, InfoBlox etc). Also, can you verify that you are getting the correct role? Maybe you are using RFC3580? Like Ronald asked, is that .1.x network from a DHCP scope and VLAN you manage? Are you sure someone hasn't plugged a linksys router into your network that's on the same VLAN wifi is using, and is now handing out IP addresses?