Software RAID striped mirrors or mirrored stripes are possible in Mac OS X. They may be 10.5 only. I haven’t tested other versions.
Simply create your striped sets using DiskUtility or diskutil, then use the command line diskutil and supply the /dev/diskx entries of the stripe sets when creating your mirror.
First run diskutil list to get the /dev/disk entries for each stripe, then create your mirror using:
diskutil createRAID mirror MirrorName JHFS+ disk1 disk2
where disk1 is the device for the first stripe set and disk2 is the device for the second stripe set.
The result is also bootable and will display correctly as a nested RAID in DiskUtility.app.