Ubuntu 11.04 Natty Narwhal will ship with 2D version of Unity for older and weaker computers
Given the fact that one of the most common targets for Linux installations is on older, 'reclaimed' hardware, it would have been surprising if Ubuntu didn't ship with a 2D window manager. Canonical's Bill Filler also mentions that "many ARM platforms fall into this category," though we're not quite sure which ARM devices he has in mind. Most low-power devices are either netbooks or smartphones -- and netbooks are Atom x86, not ARM (at least for now), and a lot of smartphones have hardware acceleration via OpenGL ES (and who puts Ubuntu on a smartphone, anyway?)
Basically, this is great news for people that want to put Ubuntu 11.04 on old laptops, netbooks and desktops with awful integrated graphics -- or simply those that don't like the 3D version of Unity! It's also great news for those that virtualize their Ubuntu installations under VMWare or VirtualBox, both of which have a hard time with 3D acceleration.
[It seems, at the time of publishing, that Bill Filler's blog post has vanished. Maybe he was jumping the gun; or perhaps something else is afoot...]