Apparently this problem isn't as common as I'd originally thought (or maybe I'm just not searching well).
I have a Dell Dimension E510 desktop. Deciding I wanted to try my hand at the beta testing thing, I downloaded and installed Karmic alpha 6. I did a clean install, formatting and attempting to create and boot from an ext4 partition. When I tried to reboot the first time after it was installed, the system goes through the BIOS, says "grub loading" then on the next line "error 15" and freezes with no way to do anything but reboot. Trying to re-install Karmic over the existing installation (so without reinstalling the boot loader or reformatting the drives) it doesn't even give me those two lines--it just goes through the BIOS then freezes on a blank screen.
I gave up and decided to try again when the beta came out. I've now tried installing Karmic beta and am having the exact same issues (tried multiple times in case it was a fluke).