For whatever reason, my iMac had some problems this week and I was forced to shutdown uncleanly. I didn't notice much in the way of repercussions until I tried to use Parallels with shared networking turned on. When I opened up Leopard's Network Preferences Pane, I noticed that the Parallels-NAT interface had acquired a zero conf IP address rather than one of the DHCP addresses assigned in the Parallels configuration. Come to find out, there was a slight corruption in my Parallels DHCP server, that needed to be fixed up before shared networking would work properly.
$ su - Password: # echo > /Library/Parallels/.parallels_dhcp_leases # killall -HUP pvsnatd
This had the effect of clearing out the DHCP leases and restarting the Parallels NAT daemon. I got the idea from the Parallels Forums.