We have just set up something similar for our training room to give people free wireless. We have used a linksys router wrt54gl (I think) and flashed it wih new firmware (
dd-wrt) to be able to run a seperate VLAN from it. That way the guys on wireless are oblivious to the office netowork and we have full security. Other than spending a fortune on a router that offers this functionality out of the box this would be the best way.