ZFS device removal is on its way but not quite in illumos yet. From Alex's blog post: "We’ll publish the code when we ship our next release, probably in March [2015], [and we will] integrate into Illumos once we address all the future work issues."

Thanks. This has been on the timeline for several years now. Nice to see that it finally hit.

IIRC, there were some other nice things that this enabled as well.