Well, I'm working on an assumption, because otherwise this should work, but don't plug your IPCop box into your router's WAN port, instead connect it to an available LAN port.
Leave the WAN port on the router disconnected. This way it acts like a WAP instead of a WAP+Router. I have a similar setup here (albeit with a DLINK Wireless router), and wireless clients are able to get a DHCP lease from my firewall/dhcp server which happens to be a linux box.

I think that given you stated you are using your IPCop box as a default gateway, it's kinda self-evident that the IPCop box already has two NICs in it, but please put that question/assumption to rest.

Originally posted here by The Grunt
Then wouldn't it be a better idea anway to put the IPCOP box in front of the router, as to keep your wireless net only sending out stuff you want? (so if someone gets access to you wlan, you can block them from doing anything from the IPCOP box)
That won't work for getting DHCP from the IPCop box, and it really also depends on what you want in terms of trust between networks. If you want to trust it as much as you do your cabled machines and are using at least WEP+MAC filtering then you won't want to segregate the two.
The other option is to divie the networks and route your wireless to your LAN, but you will have to settle for using the WAP as a dhcp server.