I know usability is more than ease of installation. But there's a point at which one graphical UI is functionally like all the others, and everyone starts arguing/focusing on a lot of little details.

Harold's point was that Linux (or Ubuntu at least) has slain the installation dragon-- an important real-world usability feature. My point was that the "Live CD" ability of some Linux distros (to run off the CD-ROM drive and not need a hard drive) provides another powerful usability feature.

With a read-only OS, users don't permanently (and accidentally) change system settings or reconfigure their desktop into un-usability. A re-boot returns the systems to the original state. A big advantage to users without a lot of technical knowledge. (Not to mention that their formerly broken pcs now worked!)

Now these are not "power users" (using old PCs and MS Windows 98, after all!), but people who just surf the net and occasionally need to write letters. So they start out by not needing about 75% of the features of a modern pc. But there are a lot of people out there like this.

Finally, there's no wizardry involved! Another great thing about Live CDs is that you can test them on your pc without changing your existing MS Windows installation. Test one yourself:

http://www.frozentech.com/content/livecd.php
(list of dozens of live cd distros)

http://www.puppyos.com/
(good one to start with)

http://www.knoppix.org/
(very full featured distro, now a Live DVD)