Thank you very much to everyone who answered this question.  I'm amazed, I
just asked this question half an hour ago and got three replies already.

To summarize the answers:  Yes, Smalltalk does arrange to share literals at
the method level, so the behaviour I noticed was correct.  And yes, it is a
good idea not to modify objects created from literals, but instead to make a
copy of them if they might be modified.  In fact it would probably be a good
idea if the Smalltalk system itself didn't allow literals to be modified.
Someone pointed out that VisualAge does just that.

Thanks again,
Adam