Yes, Apple does have built-in software RAID support. However having used it extensively I can say that SoftRAID is far better and far more intuitive than Apple's offering.

There are two main problems with Apple's software RAID. The first is notification. It doesn't have any bells, alarms, email, etc. alerts to say "hey, your RAID slices are out of sync!" you have to periodically run Apple's Disk Utility software to see, oh, the RAID went out of sync a week ago, a drive failed, etc.

In addition when attempting to rebuild a RAID set Disk Utility is as rudimentary and counter-intuitive as it gets. The biggest shortcoming of the Apple software RAID is that when rebuilding you have no idea which slice has the most current data and which fell out of sync, hence I lost a month of work on a set due to a disconnected drive, which Apple's RAID software never warned me about.

SoftRAID on the otherhand, while not perfect, notifies you in the Finder as soon as there is a sync or drive problem within a RAID set. I'm still waiting for them to implement out of the box SMS or email alerts but it still trounces Apple in this regard. The software is also much more intuitive (though the interface is still a throwback to OS 9 and could also be improved for clarity). All in all there is no comparison between the two. SoftRAID is inexpensive enough that you'd have to be crazy or broke to use Apple's implementation.