Bits and pieces about how this came to be...
Flexibility syndrome
Here are some discussions which look like the first ones to mention this concept.
The first bug
Is number 4 on the Apache Bugzilla system.
Cocoon digital archeology
Some mail datamining by Stefanomazzocchi.
Flow - the vision
Here's the first message from OvidiuPredescu about flow.
The Cocoon Wiki is born
Here's the announcement by StevenNoels.
The Original Scheme Implementation of Flow
...there's plenty of space here to add more interesting/anecdotical stuff, please do!