Mike,
You can create a new vlan ( e.g. vlan 99 ) and add four ports on your catalysts to that vlan. On the routing blade, do not add an Ip addess as you have said.
This would provide your DMZ on a private vlan.
However, you would still need to have connectivity from your internal network to this new MZ location which would replace the connection to your existing shared hub.
Is the PIX configured for one inside interface (which connects to your internal routed LAN) and a seperate outside interface for external traffic, as well as this DMZ you have created?
Please provide more config details of the topology layout if this does not answer your question.
Colin