Thanks to all for your replies -- this information is valuable and certainly a good start for me.

More practically, what do I have to do to establish a license for some software? Michael, if I'm not misrepresenting our private correspondence, you have recommended noting the license in each file I distribute, including the full text of the license in the distribution, and including a (dated) copyright notice in each file (or is this complimentary to the actual licensing?).  Is that it?

Also, do people ever change a license mid-project?  How is that done?

And what does it mean for licenses to be compatible?