I agree to Martin's patch for 2.6, because it seems to provide a more
clean solution. And, as he states, there will be time and opportunity to
discuss it more thoroughly later to possibly find another approach. As I
remarked before I know that singleton+inheritance is a controversial
subject. However the gang of four state in their book that one should
use the singleton design pattern if one needs exactly one instance af a
class and this should be extensible by subclassing ... (And there are
reasons why I'd like to have Screen() subclassable.)

Anyway, I think there are two more things to do:

1) Add a docstring to the Screen() function
2) Insert (at least) one sentence in the docs, 9.th paragraph of the
Introduction. Something like:

... so there can exist only one instance of Screen at a time. 
*Notice that consequently Screen cannot be sublassed.* 
It should be used when turtle is used as a standalone tool for doing
graphics. ...

Perhaps one should also note, that the singleton object is returned by a
function.

I've applied the proposed patch (file11645) to a working copy and I've
tested it in interactive mode as well as with turtleDemo and all the
supplied demoscripts and found it to work well.

Last, but not least: thanks Martin for your contribution.

Regards, 
Gregor