Hey pachini,
There are many posts describing how to do this function. Since I'm not sure exactly what you are tring to achieve, I'll give you a general brief run through. Under the interfaces tab, select the interface you want to bridge to another interface. So, if you wanted to bridge the servers interface to the lan interface, go to the servers interface and select "bridge with: lan". That interface will not need an ipaddress and will gray out the appropriate boxes.
Once this is complete, you will need to set up rules to allow the interfaces to talk to one another. You can limit the traffic to only DHCP only or you can completely open up the communication between the interfaces or choose something in between. If you want the two interfaces to act like a switch, open up the the firewall between the two interfaces. Go to the firewall tab and select rules. under the LAN tab, you will see a single rule (unless you modified it). there will be a plus button that will say "add a rule based on this one" when you hover your mouse over it. this is the rule that allows your LAN traffic to access the wan or any other interface. click that plus button. on the page that comes up, change the source tab from LAN subnet to servers subnet or what ever you see fit. If you want a more restrictive, give me a reply and ill tell you how to do DHCP only rule.
note: pfsense 2.0 handles bridging a bit differently. There is a bridging tab under assign interface tab to setup the bridge.
EDIT: Here is a good post on how to pass dhcp between bridges. http://forum.pfsense.org/index.php/topic,13351.0.html
Good luck,
-V
