Retrospectives are certainly a central part to the way most people adopt Agile. Similar to previous incarnations, like Lessons Learned and Postmortem sessions, one improvement is they are held frequently and not just at the end of a project or, even worse, just when something has really gone wrong. I'm a fan of them and of having them with different scope- not just focused on the last iteration, if you're doing time boxed iterations. I presented an experience report at Agile 2008 on holding release level retrospectives.
Tonight, I get to participate in a panel discussion of Borland's time in Austin at Agile Austin. It is a retrospective about a period in the company's history when we were doing a lot of work to adopt agile and what lessons we can learn from that.
Come by if you get a chance!