I am configuring the plugin Role Scoper for a site I am building. I highly recommend this plugin for anyone considering a CMS system. This plugin allows to you build on the incredibly easy to use foundation of WordPress but get much of the more granular with security rights for multiple users. This contrasts with full blown CMS systems like Joomla that start out with robust security controls but have a steep learning curve.
If you do have any questions about implementing Role Scoper though, just drop me a line.