Thank you very much!
I can now rule out my initial guess, KNOPPIX file corruption, as I have checked the md5sums from the image I use and a freshly downloaded, loop-mounted ISO image.
The results of memtest are somewhat confusing, as it aborts when run with 2x2GB modules, but finds no errors with either of them (ASUS UL30VT.
I still suspect there is some udevd bug involved, as I had similar behavior on a Dell Vostro.
Can conflicts between /etc/fstab and /etc/mtab confuse udevd? I have one, /media/sdaX is mounted as /store/local. In any case, I think restarting udevd when system is fully set up may be a good idea.