While I don't have personal experience with SLI, NVidia claims that with two cards in SLI and two monitors plugged into the main card, It can support 3d gaming on both screens, in windowed more. The caveat is "on supported games"

http://www.slizone.com/object/sli_mu...r.html#display

I can't find a complete list of these supported games on the Nvidia site, but WoW has always displayed the SLI compatible logo, even before they actually did...