I write a lot of code, and I prefer my editor window (emacs) to be 100% of all my screen space. (ideally, not even title bar, but not going for that yet...).
I usually stretch it out, but in 11.10 (Gnome), there's this really frustrating auto-maximize to a given monitor 'feature'.
I've tried forcing the size to be 2880x900 and setting the window to be non-maximizable in CCSM, but no go.
I found an Ubuntu forums post that indicated you will need to disable the "Grid" plugin in Compiz Configuration Settings Manager or "CCSM" You may need to install it if it is not installed.
I tested it and was able to drag Firefox across both of my 22" monitors. Hope that's the answer you were after.
In CCSM > Window Management > Window Rules you can set size rules which will make specific windows always be forced to an exact size and screen position, including spanning multiple monitors. I have used this in the past to run blender across multiple displays. This operates based on the window classes. You won't be able to unmaximize the window if you do this though.
Also, I just tried to enable this plugin and Unity instantly crashed. (923567)
Ironically, after Unity crashed and I restarted it manually, it failed to recognize my twinview setup, and now all windows maximize across both monitors! (923569)