Unity 4.5.4 added a Visible in backgroung checkbox to the player settings for stand alone.
What does it do?
Its not added to the documentation.
The release notes say “Graphics: Added Visible in Background option to windowed fullscreen.”, which I take to mean that windowed fullscreen (borderless window filling the screen) keeps the game running at all times, so you can have other windows in front (like IM, guides etc., which is a common reason players ask for windowed fullscreen).