My mother, a sweet 75 year old lady who loves to share powerpoint presentations with her friends, has been using Ubuntu Linux for the past 5 years or so. She moved up from MacOS 9, which she used since she abandoned Windows (because I threatened her I would not remove malware from her computer again).
So far, she has been very happy with it. She is now using Natty and quickly transitioned to the Unity shell.
Every once in a while, I log in remotely and brush the machine's teeth. Never found anything remotely suspicious.
With a bit more polish, a Linux locked to a rigorously maintained package repository could also fill this niche.