Yep, that's a loaded question and it assumes how you define a CMS. I have a 1999 text that talks about almost every type of information system imaginable (document management, decision support, knowledge management, expert system) but not a single mention of "content management system". Yet, we know in the mid to late 1990's there were companies such as Interwoven around.
I found this 2000 Gilbane Report that talks about the emerging CMS and whether it's a buzz word or something that may be around for a few years: http://gilbane.com/artpdf/GR8.8.pdf
. It loosely describes a CMS in much the same way that Ron describes it in his post."Content management" as used in IT circles is a relatively recent term that has emerged coincident with the web. It is used in many ways to refer to many different things. "Content management" is used to describe everything from Microsoft's Front Page to Interwoven's TeamSite to Akamai's EdgeAdvantage, and those are just a small sampling of products where, at first glance, it seems like an accurate moniker. Content management is a term that causes intelligent users and vendors to scratch their heads.
Finally, check out CMSWiki's history page: http://www.cmswiki.com/tiki-index.php?page=HistoryOfCMS
. It refers four different era's of content management systems starting in the mid-1970's mainframes.