Tony Chen wrote Tuesday in Seilevel's Requirements Defined blog about when requirements are "done". I have written many times about how design, implementation, and testing inevitably lead to changes to the product's requirements. Tony touches on this phenomenon:
"We prefer a model that lets the phases of the development cycle overlap such that it effectively becomes an iterative model. Once a critical mass of requirements are complete enough that the technical team can begin making design decisions (typically 20-30% of the way into the overall requirements effort) the design phase begins. Our experience shows that there will always be some design issues that impact the requirements so it is better to work them out simultaneously rather than trying to perfect the requirements document in advance."Tony is implicitly endorsing an agile product management approach, though I'm not sure he would want to use those words to describe it.