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 Knowledge Base page here.
Now there is a fix available here.
Just to clarify, this is a Windows Vista OS fix, not an FSX fix and not related in any way to Service Pack 1 for FSX.
The fix for 64-bit versions of Vista is available here.