How to make Jebba's kernel modules stay active?

I followed the instructions given in

http://wiki.maemo.org/User:Jebba/Ker..._custom_kernel

to the line
Enjoy

after having done that, I put in the n900 my SD-card (partitioned in vfat and Luks-ext3 part), vfat mounted automatically and then I used the cryptsetup luksOpen command, gave password and mounted ext3. All worked nicely.
Then I rebooted the device and although the cryptsetup command was there, the needed modules were no longer available.
So I used again the 2 x dpkg commands given by Jebba and rebooted.All worked again as it should. Reboot again made the needed modules disappear again.

BTW: I also changed /etc/fstab and the reboot put the original /etc/fstab in its place again.

What happened, why do the modules disppear and why is /etc/fstab restored?