Comment 3 for bug 884219

problem solved:

I made some unrelated changes to menu.lst then did an update-grub.
To my surprise, in the diff it showed adding the line: "initrd /initrd.img-3.0.0-12-generic"
Apparently, during the upgrade from natty, the installer 'forgot' to add this very cruical line and i hadn't noticed.
all is good now (except that boot time takes an extra 90s bcuz its waiting for the network to come up, but it does that an all my kernels, so i'll have to file a sep bug for that).