Well, as I get more and more into content management systems, one client has determined which one will be the one I get intimate with first: Drupal. It is a highly rated one (see Open Source CMS) and has lots of configurability--themes and modules. (For comparison with other CMSs, also check out CMS Matrix.)
It's these themes and modules that are going to be the challenging part. No web site wants to look like another and there are only so many themes that come with Drupal. I'll have to choose one to start with and figure out how to modify it.
Now this specific client wants a lot of customization, but does not want to pay for me to learn HOW to customize. Fortunately, I have another client that needs a CMS--only they don't know it yet. This other client is not one I'm charging money for, so this is a perfect fit: I'll learn and build the other client's site, then use my new-found skill to make money! Everyone is happy, right? Well, it means I have to spend more hours that I can't bill for. :-(