You can get my working busybox from
http://www.phathack.com/busybox.bz2 then use the following to start it on the serial port
/dos/busybox mv /bin/busybox /bin/busybox-old
/dos/busybox ln -s /dos/busybox /bin/busybox
/bin/busybox --install -s
(sh < /dev/ttyS0 > /dev/ttyS0 2> /dev/ttyS0 &)