My mother is pretty happy with Ubuntu. Give her a browser with Flash and some speakers, and she's all set. I'm pretty sure she doesn't know what 'linux' is, though. She lives the mozilla fox way of life.
I have a nice experimental result to contribute to this. I installed Ubuntu alongside my mother's Windows installation on the desktop, and a month later, found that she hadn't booted into Windows once. All she really needs is a browser, a pdf and .doc viewer and a media player. Chrome OS would be so perfect.
Yeah my old man actually asked me to install it after getting slowed down by viruses. All he needs is Chrome and VLC. The little thing pops up when he plugs in a camera too. Bosh.