For reasons far too tedious to go into, I needed to share an internet connection from WiFi to ethernet, and of course I'm using OSX 10.7 Lion. This also applies to Snow Leopard 10.6, and likely Leopard 10.5.
In theory, its easy: go into the sharing control panel select the correct options, and off you go.
However, this didn't work.
It was very curious. Client devices on the ethernet were getting DHCP addresses, so the built in DHCP daemon was working OK, but the Macs ethernet port wasn't being assigned the correct IP address.
Anyway, giving the ethernet connection a manual address of 192.168.2.1 made it all work fine.