After spending several days of pain and frustration over trying to get RAID 1 working with Lucid Lynx and GPT, I finally got it working. Maybe this will be of use to someone else.
Apparently between MSDOS partition tables and GPT, MSDOS automatically reserves a portion of the hard drive for the boot loader while GPT doesn’t. I guess the advantage of having GPT do so is that you can place it anywhere on the disc, although I would imagine you would want it near the beginning anyway.
You can follow either the graphical installation shown here or use the alternate cd to install 10.04, but the most important thing is that you leave a very small partition (bios_grub in order for the boot loader to handle things correctly. You can do this directly from gparted or run it on the command line like so:
$ sudo parted /dev/sda set
I haven’t tried configuring RAID 1 on the bios_grub partitions for each HDD, but I’m happy to leave it be as it works. Once that’s done, you can install grub manually (where sda and sdb are your respective hard disks):
$ sudo update-grub
$ sudo grub-install /dev/sda
$ sudo grub-install /dev/sdb