I have upgraded to 1.5.2 and still the same problem.
I finally remembered the sysctl kstat.zfs.darwin.tunable.zfs_no_scrub_io=1 thing which enabled me to import without the reboot. zpool status indicated that scrubbing was still going! But at a very nice rate: 1.5GBps

I have moved disks around onto various combos of cabinets, eSata, USB but no change, i.e. reboot after 1 minute on all combos.
To eliminate something in my setup, maybe someone could start scrubbing and then reboot the system and see if the problem can be reproduced.
Is there any debugging I can turn on to help you find the problem?