Full screen mode in two monitors
It is not possible to use one screen in full screen mode while having an different application on the second screen.
But it is also not true that the second screen isn't "usable". You can have different windows of the SAME application on the second display (e.g. the equalizer window in iTunes, or the fonts dialog in pages or whatever).

I would also prefer to have a second application on my second screen, but I mentioned the ability to have at least other windows of the same application just in case that someone didn't knew.