Since Logic 8 we can only open the Environment window as a floating window (always on top). The other Editing windows (Piano Roll, Score Editor, Hyper Editor, etc) are only available as regular windows.
To open the Environment window as a floating window either:
Hold down Option and choose WIndow > Environment
Right click in an empty area of the Environment window's background and choose: 'Frameless Floating Window'
in the Environment choose 'Frameless Floating Window'.
Would be great to be able to open any editing window as a floating window though!