The fault here is not with Apple's software RAID system -- which is based on the FreeBSD software RAID system and is intended for a lot more than just XServe, because XServe uses hardware RAID controllers. The SoftRAID people are lying to you in order to get you to buy their products. The fault is that you are trying to mirror the internal boot drive to an external volume. Non-homogenous mirroring is never a good idea. In your configuration you would be much better off using the external drive as either a Time Machine or CCC/SD backup.

For the record, I use a pair of 500GB Firewire 400/800 drives from Seagate with Apple's software RAID with both a G4 Mac Mini running 10.5 and a MacBook Pro with 10.5 and recently 10.6. They've been working perfectly fine like this for over 3 years and yes, I do swap the RAID between the two on a frequent basis.