Recently watched a couple of peeps from Salesforce.com give a presentation on their fanfare, reverb for the voice "agile transformation". There were many points to their presentation that made me go "ewww" and "ahhh" but what struck me the hardest was the big bang, all at once approach for the change.
My general opinion is incremental change, prove the new stuff works, and grow teams and projects organically from this. The hassle with this is the still running waterfall projects that will be running when the first agile project is started. No agile project in a normal waterfall shop will start well, it will be a complete disaster, every problem from every project will be exposed in the first few weeks. The project will be red - because project managers can only cope with three colours - and everyone up and down the food chain will hate it. The concurrent waterfall projects will be considered a haven of tranquility and good news, not like that *$#@ing agile project. Any team member who isn't comfortable in the new project will want to run back to the good ol' way and they will have the voice - from experience - to let everyone know how crappy the agile project is.
If you do all projects at once, every person and team, then everyone has a crappy time together. There is no "well running waterfall project" (ohh look at that spec, and that Gantt chart, now that's how you deliver software) to turn back to, to compare to, to contrast with. It's all or nothing. Lance the boil. I like it.