Comment 2 for bug 779213

Martino wrote 2 hours ago: #1

Also my Ubuntu 11.04 fully upgraded can't boot Mageia 1. Kernel panic here, too.

Look at /boot/grub/grub.cfg in Ubuntu and compare the (hdx.y) in the linux.... line with the one on the initrd..... line.
They should be the same - if they are not, edit the initrd line to use the same (hdx,y) as the linux line.
You will need to use an editor as root. This should fix it temporarily.
To work around this issue more permanently it would be better to move the Mageia entry from grub.cfg to /etc/grub.d/40_custom so that it will not be broken again when you run update-grub.
After moving it to 40_custom you will need to run update-grub for it to appear in the menu.
Hope that helps

This bug http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=566102
in OS-prober really needs fixing in Ubuntu :(