Yup, pretty much; if you have more than one vdev in a pool (mirror, RAIDZ, whatever), ZFS will stripe over them, varying what goes where depending on how fast it thinks they are.

I've heard reports of ZFS doing things like putting more data on one vdev because the disks underlying it had 16MB of cache and those in the other vdev only had 8MB; bit fancier than what you normally associate with RAID10.