As Chris says, change log implementation is essential, but there might be a better way to do it than the traditional change log management. Let's think of blogs and wikis as tools for administration and project management, not just applications for team collaboration.
One of the first things I ask a SharePoint administrator (at any level: team site, site collection, server admin) to do is to setup a blog to document changes. At first they all think it's a pain, but when they have the ability to search, categorize and accurately assess/access this historical reference, they start singing a different tune. Documenting the thought process that helped drive a decision is just as important as documenting the change, itself. A side benefit to using a blog is transparency; your community of users can have visibility into the process of creating, expanding and maintaining the site.
This might be worth an entire thread. I going to post it to my blog at EndUserSharePoint.com to see how others are implementing change logs.
Regards,
Mark
EndUserSharePoint.com