I ran into Dan North at the Agile conference today, and explained a little about the project portfolio book. I'm writing it because I have a number of clients who are having trouble breaking the multitasking habit (working on more than one project at one time.)
Dan said, “Oh, you want them to commit to serial monogamy project management!” I cracked up.
He said exactly the right thing. No one has to be married to a project forever–just long enough to finish an iteration. (That's why serial lifecycle projects are so hard. You have to stay married to the darn project forever.) But once you finish an iteration, you're free to marry another project.