Surprisingly enough, I've tried this script into another platform but same system. In a macbook santa rosa with an Ubuntu 8.04 partition , it works both in Leopard (Mac OS X) and Ubuntu 8.04 partitions.

So I guess the problem I have is specific of the PC I'm running it. Anyone any idea of commands that can help me checking the differences between the Ubuntu partition on the macbook and the Ubuntu partition on the PC (which are supposed to have same packages)?