Bug #570765 came up earlier this week and it's bringing up the fact that when installing Ubuntu 10.04 LTS on a system with another operating system present, GRUB2 will not show the other operating system once installed for the dual/multi-boot system. It doesn't matter whether the other operating system is Microsoft Windows or another Linux installation, but the GRUB2 boot-loader doesn't offer you the option to boot that OS, just Ubuntu.
The partition(s) of the other operating system(s) are not being destroyed and the menu entries for GRUB can be re-generated using update-grub. To address this issue, Canonical engineers were just going to add to the Ubuntu 10.04 documentation a note about this and to run the aforementioned command to fix the boot-loader. They've also been planning to release a zero-day Linux kernel update for Ubuntu Lucid, which would address the issue too by automatically rebuilding the GRUB entries upon installation of the new kernel. A GRUB2 update, which would also rebuild the entries, is also in the queue for being pushed out into the Lucid repository upon its release.
Just hours ago, however, it's been decided to take the best and safest course of action -- to re-spin the ISOs. However, as it would take two days to re-spin all of the ISOs, which would then push the release into May (Ubuntu 10.05 LTS?), they have decided to just re-spin select ISOs.
Read more: Phoronix