I'm glad I found this post. I currently have a machine with 3 SATA drives running sda: WinXp; WinXPMedia Edit.//sdb:SUSE10.0; Ubuntu 6.10RC and Kubuntu 6.10RC. I had Ubuntu CE on sdb also yesterday.
I was booting them all from the Grub entry on the MBR, with the default Kubuntu6.10RC. I had intended partitioning sdc for several Linux installs, as FedoraC6 is due out next week and I wanted to try it. I'm going to change the installs on sdb shortly, as I want to add Xubuntu to the mix.
I didn't realize that there was a problem with booting with GRUB installed into the MBR. What happens to GRUB that causes problems if it is located in the MBR? I need to know before a problem comes up. The installer always recognizes the other installed OS(s) and always tells me that there shouldn't be any problems if it puts GRUB into the MBR of sda.
GRUB updates the config when a kernel is updated which is handy, I think.