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Still, the challenge might be harder than you think. Did you see the recent Google video where they asked people on the street what a browser is? Less than half the people knew (most answered "Google" when asked what browser they use).
A close friend, my mentor, and some other other non-techy people I know do not know the difference between the operating system and the browser. Over the phone I have to ask them what it looks like because as far as they're concerned, they're always in "Windows".
So you need to make it as simple as possible. Strip down the admin interface if possible, create a desktop shortcut that takes them straight to the admin page.
Wordpress is about the easiest, but I've been surprised how hard it is for people who are not web-savvy to use. If they're on forums and Facebook and all that, it can help.
There are some that use a somewhat different model (I think if Wordpress, Joomla, Drupal, all the PHPNuke successors as having the same fundamental UI concept in terms of how you add a page).
Some exceptions would be ModX and Concrete5.
For Drupal 7, there's a big push for usability and they've started doing usability audits as part of each release. This was motivated by usability tests that showed that most users were utterly flummoxed by Drupal 6. I don't think it's just a Drupal problem.
It's really a lot more complicated to use these systems than we think.