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A big project like this deserves a talk page, so I'm just going to throw out a few ideas for the article. Time permitting, I hope I might be able to follow up on some myself, but other contributers might be more qualified to do so.
The original falling out of a number of JBoss developers that led to Geronimo is probably worth mentioning, and might shed some light on some of the philosophical differences between Geronimo and JBoss.
The rise and... buyout of Gluecode should also be mentioned in its proper context here, since IBM's involvement is closely linked to their taking over of Gluecode.
Those are the big two things I think this article could use, although if someone has anything profound to say about the commoditisation of the application server market with the emergence of products like Gerominio changing the playing field for established products like WebSphere (non-CE) and WebLogic, that would be cool to have too.