Sorry to be the bearer off bad tiding's but as off yet there is no support for spanning, you can use the extend my desktop feature to run 2 monitors (wich will work for supreme commander but not ms flight simulator for ecsample)
Whether it will be supported in future is (I think) down to microsoft as it is to do with the os not nvidea driver's.
Whether ati's eyefinity tech (available on 5870 ect) works with win 7 is unknown, but if it dous and n'videa dousn't follow suit, I will probbably switch to ati.....or back to win xp (as I havn't actually perchased win 7 yet I'm just on the rc)
If you're listening microsoft/nvidea, time to pull your finger out and fix it, or risk loosing million's off potential upgrades to win7.
(the net is full off people complaining about this, + nearly every flight sim buff i know off)
I did a quick google and there is a way to do it using 3rd party softwaire alltho its not tested in win 7.
There is one workarround but it takes additional software to make it work. Note this is only for Nvidia display adapters using latest nvidia drivers in Vista. I do not know of the software and this workarround work in windows 7. I also use World of Warcraft as the example game/software since it's what i was trying to get spanning in vista at the time. Step 1. Create a custom resolution in the nvidia controll panel. it should be same height but twice the with *assuming both monitors are same size/make/model. Step 2. Purchase/install Ultramon. Step 3. choose a hotkey for you to use to cause any window application to span both monitors. Step 4. open World of warcraft. Under video in resolution section you will see your custom resolution available. select it. Make sure you selected windowed mode and maximized. step 5. Push the hotkey *i used ctrl-alt-numpad 0 * Enjoy spanning both monitors with the application. WARNING: Ultramon has not been tested with windows 7 and i do not know if there is a 64 bit version. This should work with any software that allows you to play/use in Windowed mode, maximized. some games and applications will still have a border when used in windowed mode. Another option is to purchase a Matrox Triplehead to go *assuming you wish to use 3 monitors instead of 2*
hope any off this was usefull!