If Ubuntu can get away with their plan, and not have their key revoked by the capricious gods of key revocation, then it sounds like a better approach to not require signed kernels. Or am I getting confused, and this will apply only to machines with Ubuntu preinstalled, while installing Ubuntu on a stock UEFI machine that originally shipped with Windows will still require all the shenanigans that Fedora goes through?
A machine that comes with Ubuntu pre-installed could still run Fedora or Windows or any other operating system, since the Ubuntu bootloader can surely be configured to boot other Linux distributions or OSes. It is not really necessary to include Microsoft's key or anyone else's. Indeed, it might be wise for Canonical to spin out their initial signed bootloader as a separate project, becoming the upstream for many other distributions. Then hardware vendors need include only a single key in order to boot pretty much any Linux distro.