ok, thanks.
you mean if you go full screen in Lion that other displays will black out? -- if so, surely this is a bug
Not really. For example 2 screens where the "desktop" spans both. If "full screen" corresponds to just as big as the desktop you now have the app's interface spread out over 2 screens. Again put it into a context for example of a photo editing app. Usually the "working area" is in the middle of the screen". However, in this expand to fill both screens, mode the middle of the screen is split over two monitors. That is probably not what the user wants.
The user wants to
1. Hide every else but this app. (which blanking out the other screen does)
2. fill the app to the size of a screen (probably.)
I suspect you are thinking "Oh the OS can tell the app where the screen breaks are and the app automagically flows platelets and working areas to one side or the other of the physical breaks.". I doubt this "autoflow" is going to manage to flll the area of both screens with app artifacts *and* tiptoe around the physical breaks without using lots of "blank space" just like the default does.
There is a fundamental conflict between "consume the screen" (assumes just one) and having a multiscreen set up. The latter lends its self to user managed windows. The former doesn't.
does SL have an equivalent feature? - if there is i'm unaware of it
It was apps specific ( just like in Lion since have to use new libraries. It is more question now of multiple developers not reinventing the wheel).
There approaches to full screen and two monitors. For example Lightroom.
http://www.youtube.com/watch?v=AKEbAkSOXXY
I just don't see Apple not doing that in version 1.0 of the library as a bug. If there are sufficient requests for it, it will come later. Even this is limited though. What if have 3 or 4 monitor... what goes in the one beyond the second ?
Frankly, every app doesn't need "full screen mode". I really think Apple is blowing it by blurring the notion between "app library" features and the OS. Lion doesn't make every app full screen any more than cover flow (in earlier releases) made every app album flipping exercises.