Making of the initrd is part of the kernel %post script. If, for some reason, you're using a 3rd party module in your current kernel, that initrd thinks is required, then you should have gotten a message about the %post script failing. In all the tests I did, I use rpm manually or with yum, and the initrd.img was created.
Can anybody else duplicate this failure using apt?
No, it worked on all the machines I've used it on...
-- Eric Rostetter
