If you are DHCP configuring your interface(s) but won't always be attached to a network with a DHCP server, then you can tweak the settings to reduce DHCP boot up delays signifcantly.
By default DHCP will wait 60 seconds before timing out when trying to get a DHCP lease. If you boot while the ethernet interface it is not connected and the interface is set to be DHCP configured, it will delay the time it takes to boot up. Open /etc/dhcp3/dhclient.conf in your favourite editor and find the settings below. Then change them as shown.
timeout 5; retry 2; reboot 3; select-timeout 3; initial-interval 2;
These should be fine for most network environments (including wifi), but if you fail to get a DHCP lease then just increase the timeout and retry appropriately.