OP here, I learned a lot from asking this question. Specifically that the Deadlines I have seen fail, were not due to Agile, but failure to implement Agile. Agile takes buy in both from the Customer, managers and dev team. This also takes education on roles and expectations. Without buy-in, you lose the check's and balances and end up with dev teams on a death march, or product deliveries not providing value for the Customer.
Ron Jeffries has a great post on this topic that I highly recommend http://xprogramming.com/articles/jatmakingthedate/