Since you were knowledgeable enough to run ifconfig, I've made assumptions about your ability to perform the following actions.
Check the Airport card by creating an ad-hoc network on the Mac and then attempting to connect to it from another computer. [Open /Applications/Internet Connect; select Airport; choose 'Create Network' from the dropdown menu.]
Check the wired Ethernet interface either by running a cable directly from the Mac to your Internet gateway or to another computer (Macs automatically adapt wired Ethernet links to suit the connected hardware. That means there's no need to use crossover cables when connecting to other computers without a switch being present.)
Making successful connections eliminates a hardware fault.
With the iBook unattached to any network sources, make sure the loopback interface is working (for both IPv4 and IPv6). Using Terminal.app,
ping -c5 localhost
ping6 -c5 ::1
Check the integrity of the firewall. Look for entries which could deny access to the DHCP server.
sudo ipfw list
Try to establish a connection as you normally would, and then see if the Mac has any information about its DHCP status.
ipconfig getpacket en0
ipconfig getpacket en1
Barring any apparent faults which you can remedy, post your results here and I'll do my best to interpret them.
Before doing that, however, I recommend you try the Mac's GUI network troubleshooter. You can launch it either via Terminal.app...
open -a Network\ Diagnostics
...or by changing directories in the Finder to /System/Library/CoreServices and double-clicking on Network Diagnostics, or by opening System Preferences, selecting 'Network,' and clicking on "Assist Me" at the bottom of the preference pane. The application will guide you through the troubleshooting process.
Verify that the Mac's Ethernet addresses aren't listed in a MAC address filter in your DHCP server, or, if they are, that any enforced restrictions don't involve DHCP leases. If your router has a persistent firewall, ensure the Mac hasn't been blacklisted.