Linux.by

a. mknod --mode=0640 /dev/ttyLT0 c 62 64
b. Change owner and group owner to match /dev/ttyS0
(Debian users: set group to dialout)
c. Create symlink /dev/modem to it ('ln -s /dev/ttyLT0 /dev/modem')

Edit KERNEL_DIR variable in Makefile to reflect your 2.6 kernel dir.
Then just run make. You will get ltmodem.ko & ltserial.ko.
Copy them to /lib/modules/`uname -r`/other/ dir and
run depmod -a.

Edit your modprobe.conf to load ltserial (not lt_serial)
for /dev/ttyLT0.

My modprobe.conf contains following lines:
alias /dev/modem ltserial
alias char-major-62 ltserial
alias /dev/tts/LT0 ltserial