THIS APPLIES ONLY TO GRUB legacy
After installing Ubuntu 8.10 I couldn't boot the new ubuntu. It took me a day to discover the problem. What went wrong:
I've got a multi boot system. Windows XP and Vista (though almost never used :-) ) Ubuntu 7.10, 8.04 and 8.10.
I have a dedicated Grub partition. As explaned here: dedicated Grub partition. That was the problem. Since Ubuntu 7.10 ans 8.04 use the Ext3 File system. Their Grub version does the same. Ubuntu 8.10 does use a slightly different vesrion of Ext3. The preciese details are not known to me, but in order to be able to boot in this configuration, Grub in the MBR must be able to read the filesystem that Ubuntu 8.10 uses. It's not Ext4, but the older Grub (from ubuntu 7.10) couldn't read it.
I solved it this way.
I took my copy of the(most recent) super Grub cd, booted with it, and used it to boot the new intrepid ibex ubuntu 8.10 partition. From that intrepid session I reïnstalled the dedicated Grub partition as described here. Where the important way is that all files except menu.lst are copied with nautilus, thunar or whatever filemanager from /boot/grub to your dedicated grub partition /boot/grub. After that start grub in a teminal, make your intrepids partition the root (in my example that is /dev/hda5
So in Grub I did:
and if that's done, it's time to check the various menu.lst files, but your grub in MBR is capable to boot the newly made partition of Ubuntu Intrepids Ibex 8.10.