I have such problems (ibook G4), too.
Sometimes this procedure helps:
killall dhclient rmmod -v bcm43xx modprobe -v bcm43xx ifconfig eth1 up iwconfig eth1 essid $YOUR_ESSID ifconfig eth1 up dhclient eth1
check, if you have an valid accesspoint (MAC is not 00:00:00....) with iwconfig before you try dhclient
I have such problems (ibook G4), too.
Sometimes this procedure helps:
killall dhclient
rmmod -v bcm43xx
modprobe -v bcm43xx
ifconfig eth1 up
iwconfig eth1 essid $YOUR_ESSID
ifconfig eth1 up
dhclient eth1
check, if you have an valid accesspoint (MAC is not 00:00:00....) with iwconfig before you try dhclient