Lars, I think you might have to differentiate between two different levels of reliability here, not to mention SCSI vs. IDE.
The first is 'stay up at all costs'. In my experience all bets are off if one or more of your IDE disks connected to the mainboard are in questionable condition. Wether the system will stay up or reboot reliably from a BIOS/hardware point of view if one of the master IDE disks decides to call it a day is anyone's guess.
Thus for a hosting compagny with hundreds of disks, the use of SCSI hotswap RAID controllers will help ensure undisturbed sleep, because the, possibly partial, failure of one disk in a mirror array has predictable results: None in the near future. The admins will be notified by the monitoring software, and can replace the failed disk when convenient.
The second part is 'filesystem integrity'. I am looking forward to playing with FreeBSD's very capable Vinum Volume Manager. If my experience with the resilience of the UFS *BSD filesystem with enabled softupdates in the face of power failures and end user ignorance is any indication, then I will be very surprised if this 'software RAID' system is anything but very reliable from a data integrity point of view. I do not have much experience with Linux software RAID though.
So by using software IDE RAID/Vinum I would expect not to loose much data up to the time of the disk failure, but I would expect to have to manually unplug the dead/dying disk to bring the system up again. Wether that is acceptable for a given application will have to be balanced against the cost of a hardware RAID SCSI array.
PS: I will not be home much for the next five days or so.
