I just joined THG because of this. I too was wondering if there was any way to bring back up the raid if you were to format the OS drive. I have run a test where I set up the raid 5, threw some files on there and then formatted the OS drive and see if the raid could come back up. After the format I could not get XP to see the raid. If anyone else has tried this please let me what they did different if they got this to work. I have been running on the XP raid 5 for a few months and have been happy with the results, but like most of you said, this would pointless if you have to have another system set up, etc. If there is no way to recover the raid after a format, I will look into a hardware solution.
System specs
AMD 64 2800
ECS K8HT-1600 board
1.5 gig ram DDR400
Seagate 120 OS drive w/ XP (SP2, and all the other updates)
4 X 200 gig Seagate drive for Raid 5
Also another note for those wanting to try this, please make sure you run your system on some type of battery backup system. If the power were to go out in your home and your systems doesn not shut down properly, the raid 5 will have to regenerate itself. I've had this happen several times which shows it works as it's suppose to but it is a hassel because if you have a raid the size like mine, it takes usualy a night to rebuild the raid to "healthy" status according to the disk management.