Okay, so I got this configuration:

# zpool list
NAME    SIZE   USED  AVAIL    CAP  HEALTH  ALTROOT
rpool    68G  7.18G  60.8G    10%  ONLINE  -
# zpool iostat -v
                 capacity     operations    bandwidth
pool           used  avail   read  write   read  write
------------  -----  -----  -----  -----  -----  -----
rpool         7.18G  60.8G     31     14   814K   528K
  mirror      7.18G  60.8G     31     14   814K   528K
    c1t0d0s0      -      -     12      8   509K   530K
    c1t1d0s0      -      -     13      8   510K   530K
------------  -----  -----  -----  -----  -----  -----

But I don't want a mirror, I want space!

This should work, but it doesn't:

# zpool detach rpool c1t1d0s0
# zpool iostat -v
               capacity     operations    bandwidth
pool         used  avail   read  write   read  write
----------  -----  -----  -----  -----  -----  -----
rpool       7.18G  60.8G      8      6   367K   383K
  c1t0d0s0  7.18G  60.8G      8      6   367K   383K
----------  -----  -----  -----  -----  -----  -----

# zpool add rpool c1t1d0s0
invalid vdev specification
use '-f' to override the following errors:
/dev/dsk/c1t1d0s0 overlaps with /dev/dsk/c1t1d0s2
# zpool add -f rpool c1t1d0s0
cannot add to 'rpool': root pool can not have multiple vdevs or separate logs

Ahh, I should have done some light reading, from ZFS Troubleshooting Guide:

You cannot use a RAID-Z configuration for a root pool. Only single-disk pools or pools with mirrored disks are supported.

I was thinking of reinstalling, but no, I'll go with two different pools. By the way, I understand the need for redundancy, but I'd prefer more spindles here.

# zpool create tank c1t1d0s0
invalid vdev specification
use '-f' to override the following errors:
/dev/dsk/c1t1d0s0 overlaps with /dev/dsk/c1t1d0s2
# zpool create -f tank c1t1d0s0
# zpool iostat -v
               capacity     operations    bandwidth
pool         used  avail   read  write   read  write
----------  -----  -----  -----  -----  -----  -----
rpool       7.18G  60.8G      5      4   246K   255K
  c1t0d0s0  7.18G  60.8G      5      4   246K   255K
----------  -----  -----  -----  -----  -----  -----
tank        73.5K  68.0G      0      9  18.3K   165K
  c1t1d0s0  73.5K  68.0G      0      9  18.3K   165K
----------  -----  -----  -----  -----  -----  -----