Hi,
I did this a year ago. The file that Opie/Qtopia looks for is /etc/pointercal. You find this out by telneting in and running qpe from that session - the first thing it complains about is that it can't find this file. Simply having that file present will stop Opie from recalibration but if you don't have valid values in the file then Opie will spit and restart everytime you touch the screen. My pointercal file has the following data in it "21219 0 120027 0 16452 -600432 65536" with no newlines at all. Putting information into the file is probably a better idea.
I've since replaced my digitizer with one for a Dell Axim. It works fine and is almost like brand new apart from where I clumsly scratched the LCD. Go to the follwing link for guidance on how to replace your digitizer -
SL5500 digitizer replacement. I got it all fixed for 35ukp. Only slightly more than Sharp wanted just to assess the damage!
Hope this all helps and please let me know if you aren't having any luck. I went a month wiithout using my Zaurus because I couldn't find this information! It was only when I was playing around with it that I found this out.
Cheers,
Daniel.