This is a common issue which often comes up. Our former UX architect even had some image with the concept (which he called a "polaroid"): http://mmiworks.net/pics/blog8/lgmpolaroidL.jpg
And for the full post: http://blog.mmiworks.net/2009/09/gimp-single-mode.html
This said, it also raises another discussion we had yesterday on IRC: is the distinction between the single and multi window(s) mode really necessary? I feel that we are forcing users to choose between 2 extremes.
Moreover some SWM users seem nearly angry at us that this mode is not the default (and I feel that if we changed the default, some MWM users would get angry). Not that I really care at who is angry. But I care at what will make GIMP more usable. And I am not sure having 2 antithetic extreme modes is the solution.
So why couldn't we have a single window mode type UI and still be able to detach images out (not only as polaroid, but also to work on it; right now only docks can be detached). For instance someone could work on several screens, would want to open one image on one screen and the other on another screen for instance and still prefer the SWM UI.
Just some food for thoughts. :-)