at the end of /etc/devfsd.conf:
LOOKUP          .*              MODLOAD

in /etc/modules.conf:
alias /dev/usb*         usb-hosts
probeall usb-hosts      usb-ohci acm


-------
RK:
Under Lfs-3.1, I had to make a device to handle the USB mouse I have.

(as root)

cd /dev
mkdir input
cd input
mknod -m=rwrw mice c 13 63
cd /dev
ln -s /dev/input/mice mouse

reboot