Quite some lively discussion of this question is going on at Theserverside.net. Contributions cover the entire range from pro to contra, and astonishingly, very few posts are flame bait. Worth reading, whatever your attitude towards the topic may be.
And another topic related to Open Source is the change of licensing recently undergone by SpamAssassin. This was a long and painful process - more than 100 developers and contributors needed to be contacted and convinced to assign copyright to the project before the licensing change was made. We here at #develop ask for a Joint Copyright Assignment from all contributors since inception of the project. This episode now shows that our doing so provides other benefits besides those outlined in our wiki. Note to all out there who consider an Open Source project: think of copyright and attribution issues even before you start your project. Later modifications *are painful*.