I went through the Natty upgrade process yesterday, and everything worked perfectly up until it came time to reboot, at which point it came up with a grub error saying
So I booted from a Natty LiveCD and purged and reinstalled grub-pc and grub-common (following some instructions on the forums for dealing with this problem)
error: symbol not found: 'grub_env_export'
That got it booting ok, but with a really crappy low resolution plymouth screen. After reinstalling the System76 drivers and playing with plymouth-manager, i've got it to the point that I see the proper plymouth animation at it's proper resolution when I shut down, but when I reboot there's nothing at all until the login screen.
(I don't think this is related, but the other minor issue i've run into is the background of my KMenu launcher is garbled - see http://oi55.tinypic.com/96kkt0.jpg for what I mean)