Android USB tethering to an OpenWRT router
Use OpenWRT 10.03 or later. Install the following modules: 

kmod-nls-base
kmod-usb2
kmod-usb-core
kmod-usb-net
kmod-usb-net-cdc-ether
kmod-usb-net-rndis
kmod-usb-ohci
kmod-usb-uhci


Then edit /etc/config/network. Under the WAN configuration, change option ifname eth1 to option ifname usb0. Then enable USB tethering on the phone and reboot the router.

This article has some information about how to get the configuration smoother so you don't need to reboot the router each time you want to tether. This article has a simpler way.