FYI, @mdboom, the discoverability of `wcs.sub` is somewhat poor; it's not indexed from the [WCS frontpage](http://astropy.readthedocs.org/en/latest/wcs/) but only from the [WCS class documentation](http://astropy.readthedocs.org/en/latest/api/astropy.wcs.WCS.html#astropy.wcs.WCS.sub)