For what it’s worth, I’m using a GTX 295 with a 8800GTX on the same system with Vista, and Cuda sees both devices. It was a bit fiddly getting it to work, since you need a virtual monitor on each GPU for Windows to load the drivers. Thankfully, with the latest drivers, this is fairly easy.
Go into Display Settings and it should show a list of all monitors, including a virtual monitor for each card that isn’t actually plugged into one. For each of the virtual monitors, check the Extend the desktop onto this monitor box and hit apply. Now Cuda should be able to see all of your GPUs. The only problem is that you can end up with windows and such displaying on the virtual monitor, and hence invisible, but you can live with that.
Finally, it’s worth noting that cuda doesn’t automatically extend across GPUs - you have to create a device context for each physical GPU. An annoying thing to port, but certainly doable.