This has real potential. I'm interested, so please report back if you like it. http://www.lightneasy.org/
RazorCMS - http://www.razorcms.co.uk/ - have tried it and liked it a lot. The developer is constantly working to improve it and the results show. NanoCMS - http://nanocms.in/ - another nice one too. I played with this one for a while and created some small websites. Unfortunately my (small useless) host at the time got hacked and the sites were messed up, so I packed my bags and moved to another host and rebuilt the sites using Drupal. But it's a nice flat CMS to work with. There are quite a few others out there but these are the ones that I have worked with.
I just previously checked nanocms and it was pretty young. I tried simplse cms too, but didn't like it..let me check yours now.thanks..
There is a reason flat file cms are not widespread. Creating a database in Cpanel using the wizard takes a couple a minute or two at the most. Wordpress can be installed in only a few minutes after that - even first timers can install in in half hour. The only problem with Wordpress or any Database driven sites in my view is with Faulty Hosts who mess up your files on their end and screw up your site. So Back everything up! Or find a REAL host.
I am interested on a flat file cms because of my host(which is free) is giving me more space on files rather than database..I have 300mb of file and 10mb of database...
You can also use Pure SEO CMS. Flat file CMS systems have lighter, faster pages that don't slow like, Joomla etc, with high traffic. They are also more reliable and secure than online database programs and generally much quicker and easier to set up. You can't let lots if users update them or from different locations though very easily though.