That seems to be a question on many OS X users' minds these days.
It seems "Fullscreen" for native OS X app's is meant for single display systems such as MacBook's and iMac's as it maximizes screen real estate for smaller displays. On my Mac Pro running 2 24" LED LCD's, it places a linen background on the second display. Interestingly, in app's such as "Pages", I can use the second display to arrange other windows for "Pages" while keeping the main document maximized (such as tool bars/windows). This still doesn't benefit those who would like to use fullscreen on one display (it is different than maximizing the window as it takes full advantage of the display) and fullscreen or simply having use on other displays. Since 10.7 Lion, we [developers] have been filing this as a recognized bug left and right, to no avail. It's a simple fix, I do not understand why Apple refuses to remedy it.
