Under some circumstances (whatever these are - I have no clue), ubiquity - Ubuntu's installer - fails to generate the initial ramdisk file (initrd). This bug is already known for a long time. One solution is to enter the just installed system from another (working) system and let it generate the initrd. Another solution is to get the initial ramdisk file from somewhere else. Either from another computer where ubiquity did it's job correctly, like I did, or from some other resource. The best way is to check if everything worked accordingly after installation and in case the initrd file (located in /boot) is missing, you can for example download it from here:
http://temp.mkcs.at/linux/ubuntu-1104-natty/initrd.img-2.6.38-7-generic.bin
