There has been a known issue with Vista and multiple monitors, where trying to go full-screen with any application did not work. A black or flickering screen was the result. Fortunately running windowed was no problem.
Customers who called in were sent to the KB page here.
Now there is a fix available here. And the KB page has been updated to reflect that.
Just to clarify, this is a Windows Vista OS fix, not an FSX fix and not related in any way to SP1 for FSX.
Edit: Here is the link for 64-bit Vista.