Over 6 months into Vista-ville, and I've discovered that PerfectDisk 8 (a Vista-compatible NTFS defragger, one of the highest rated) and nVidia nForce 4 (for AMD) RAID don't play nicely with each-other when using RAID 0+1. I had similar issues with XP, so I believe that the issue is more related to the drivers for the chipset than OS, so I've re-built my main system as a pair of RAID-1 volumes instead of one big RAID 0+1.
The symptoms are (and have been) warnings from the "MediaSentry" (the nVidia name for the RAID alert feature) that one of the drives in the set has failed. Rebooting the system shows a degraded state for the array, plus a second array (with a single drive) as failed.
Turns out that destroying the failed single-drive array and putting it back into the degraded array is all that the system needs to get back to nominal.
However, as I experienced with XP, if you wait too long to do the repair, a second drive from the array can "fail", and if it happens to be the mirror-partner of the first failed drive, then you loose the entire RAID volume.
And the reason I suspect involvement by PerfectDisk is that the symptoms only occurred after I went to nightly defrag runs. When I stop those automatic runs, the symptoms completely disappear.
I've run regular, overnight PerfectDisk runs on other configurations (single drive and RAID-1) without ever having an issue, so I don't blame it for the problem; I just think that the amount of drive activity being shoved down the throat of the nVidia driver causes it to choke—literally—and miss one or more timing periods for the array.