Have you ever wanted to do this or have tried to do this? How did you go about creating the script, did it all work out well? I am very interested to learn about how this went for people, I am fascinated in people who create scripts like this as they seem to work so well. I would love to hear from a few people about the problems they faced and how they fixed these, and also about the general script creation.
I usually start with being bored or having an idea, then I just dive into Dreamweaver and code. I usually lose motivation after about 60% completion though. I have a folder full of half-finished blogs, cmses and other minor stuff. Heh. Some of those I do finish, are usually used for my own sites or just given away to my friends gratis (no support whatsoever)
I wrote a CMS from scratch called Pyrophobia. I even hosted it on SourceForge at one point and had a userbase of around 100 people. Sadly I grew tired of maintaining it and let it die a few years back. You can still get the source code over at sourceforge (search "pyrophobia" if you're interested, its nothing special though) Doing this project taught me a lot of my core PHP skills and formed the foundation of my programming knowledge as it stands today. If you've never written a large script before, I would thoroughly recommend it!
That's a waste, I've always wanted to have a community (however small) of users using my script and me supporting it. But with my kind of motivation, I doubt I can pull it off.
When we made a forum script, it went well for over a year, many users and people using it, then we tried to get clever and re-do the script, it failed, we lost our motivation and moved on from it Shame really.
I have coded custom CMSes for many of my clients. They take normally take a week to a month to finish.