NAT won't start on 10.7 Server Mac Mini I'm new to OS X Server (started with Lion) and I'm learning a lot by trial and error so bear with me please!

I'm running a mini with 10.7 server who's currently sitting behind an Airport Extreme Base Station. I wanted to let the server do DHCP and and let the AEBS do NAT but apparently the AEBS doesn't allow it (I've tried this workaround but it didn't work for me) so the plan was to experiment with setting the AEBS as a dumb bridge and let the server do DHCP and NAT.
The DHCP portion seemed to work fine (Server Admin tool is showing the DHCP clients) but NAT just won't start.
When I click on START in the GUI or try from the command line (serveradmin start nat), I get this:

Aug  5 00:18:24 helios servermgrd[4310]: servermgr_nat: nat config:
                    Error:Could not create Mobile Internet Sharing config}
OP Here.
I got it working by making my mini the gateway and making the AEBS a dumb access point (plugged into an internal-facing ethernet port), and also enabling the internet sharing on the mini.