> How does the user indicate he wants the keyboard focus
> to attend to a particular place?
> What moves it?
> What would not be allowed to move it?

I would stick with existing practices of common operating systems,
such as Mac OS X [1] or Aero [2]:

- there is at most one focused element at a time.

- tab: moves the focus to the next item in a sequence (except for
text-oriented controls that want tab characters, where alt-tab should
be used to move to the next one).

- shift-tab: moves the focus in reverse direction (except for
text-oriented controls, where alt-shift-tab should be used to move to
the previous one).

- ctrl-tab: moves to the next window.

- shift-ctrl-tab: moves to the previous window.

- enter: presses the default button (e.g. accept a method).

- esc: presses the cancel button (e.g. cancel a method).

Of course the exact keys to be pressed should be discussed (maybe
freely configurable), this is just an example. These stories
(features) would already increase my productivity in Squeak heavily.

Lukas

[1] http://developer.apple.com/documentation/UserExperience/Conceptual/OSXHIGuidelines/XHIGUserInput/chapter_11_section_3.html
[2] http://msdn2.microsoft.com/en-us/library/aa969768.aspx

-- 
Lukas Renggli
http://www.lukas-renggli.ch