Comment 1 for bug 348039

I don't have a good general, user-friendly solution for this, but based on my own experience, what I would suggest is that you designate a single install of Ubuntu / grub to be the "master" (installed to the MBR), and manually configure it to chain-load to the boot partitions of any other installations - and then when you're doing your additional installs, force grub to install its boot block to the boot partition instead of to the MBR.