Many software RAID solutions cannot be used on boot partitions for the simple fact that the BIOS (or whatever boots your computer) has to be able to read it in order to get to the OS where the RAID software can run, and, obviously, the BIOS doesn't have the ability to work against a RAID. This is irrelevant when talking about hardware RAID, since it will be presented to the computer as a standard storage device.

There are some software RAID solutions where you can mirror your boot drive. The BIOS boots off of one of the mirrors, and once the OS loads the RAID software it attaches the other mirror(s) to the "main" mirror. Not all of them can do this, though, as it requires that the mirrors be indistinguishable from unRAIDed partitions, and many keep metadata on the same partition as the actual data.