since i installed debian along side ubuntu my ubuntu installation (which was working ok) crashes onto a maintenance shell during boot up complaining that apt get is not installed and several other problems too.
i can boot in after pressing Control D but i'm worried i've damaged ubuntu and to add insult to injury debian is proving very troublesome too!

did you install debian in to the same disk partition as ubuntu is installed in? if you did without formating the partition you could have a mishmash mix of both distros causing serous problems...

i have three distros in a multi-boot setup two gentoo & one slackware, and all three live side by side without any problems...

another thing to be aware of is the ubuntu developers did something really stupid to their system = they implemented UUID for identifying disk partitions in both the grub bootloader (/boot/grub/menu.1st) and /etc/fstab which could also be causing all or part of the problem (something you may want to look in to)...

Debian and Ubuntu are more or less the same thing anyway. Why not dual-boot Ubuntu with something different: Slackware for instance?

I'm running 5 distros on the same HD with Ubuntu as primary and the same thing happens to me constantly. I discovered that if I install the other distros first and Ubuntu last then modify the menu.lst in Ubuntu (root, chainloader +1 etc.) all goes well until I do a lot of work on one or the other and then Ubuntu crashes again. Like you say, it's not a big crash - and typing 'exit' in the shell gets it to boot but it is frustrating! I'm going to change things around and put Kanotix as the primary and put Ubuntu in the back somewhere and see if that works for now.

The correct solution, I believe, is to put Grub into its own partition and then install grub into root on each of the distros put on the HD.