The concept of the 'last lecture' is meant for professors to really think about what they would like to tell in their last lecture. A few 'last lectures' already have been given. A really powerful concept, isn't it? Forget all the humbug and daily routines: you have just so little time to deliver your last lecture. It better be good!
Recently, professor Randy Pausch really gave his last lecture. This wasn't just an exercise for him, since he has pancreatic cancer and only has a few months left to live. Just take a look at the video for a few minutes: it is really impressive. The video is brought to us under a Creative Commons license ("The video and transcript will technically be under a "creative commons" license, which will be a legal way of saying that you may use them for non-commercial purposes, so long as you don't alter them, and you give attribution. Please bear with me; figuring out that legal stuff is secondary to my other priorities.")
Update: there is also a great video by Pausch on time management (made in 1998, without Powerpoint....).