List DHCP leases held from client Does Mac OS X Snow Leopard store complete information on the client DHCP leases it holds on Snow Leopard Server?  It is apparently not recorded in /var/log/system.log — although this information is logged here on Tiger.
I can get the DHCP leases granted from sudo serveradmin fullstatus dhcp; this is the other end to what I want.
The issue arose when I managed to cripple the machine so that, while it was quite capable of accessing the local network, it couldn't play its part in receiving a fresh DHCP lease, and so the router wouldn't talk to it on any channel other than 80, isolating from anything outside the local network.  I fixed the issue by rolling back the state with Time Machine, but I'd like to know how to troubleshoot this issue in future.
Postscript
Which it did, and being able to verify that no leases had been generated in the past week (by looking at the date of the leases directory in the output of ls -l /var/db/dhcpclient) was useful in debugging the situation, so kudos to Harv.  You can get more information on the individual leases by looking at ipconfig getpacket $INTERFACE, but not the date of when the last lease was issued/deleted.
What happens is that the ipfw firewall was blocking the ports used by DHCP.  Switching off the firewall allows DHCP leases to be granted.  Now, why these ports were blocked is still a mystery to me.
I don't think you can find lease information from the client end, I believe it's stored on the server end.  I don't quite understand, though, your predicament.  You managed to cripple your network connectivity somehow?

Try /private/var/db/dhcpclient/leases.