If you have it in server only mode, you will loose the firewall functionality of the SME server, so if it's in server mode i would suggest you just use port forwarding, and not to place it in the DMZ, but if you change it to server/gateway mode, then it will be fine as long as the external nic is the one in the DMZ.
if you do a search for IPCOP here someone talks about placing an SME in a DMZ, have a look.
hope that helps
cheers