Ok, so I threw a zpool status on my array today (as I do at least once a 
week) and got this message:

status: Some supported features are not enabled on the pool. The pool can
 	still be used, but some features are unavailable.
action: Enable all features using 'zpool upgrade'. Once this is done,
 	the pool may no longer be accessible by software that does not
         support the features. See zpool-features(5) for details.

Now sounds to me like I should just do the zpool upgrade thing but I 
figured I should check with the ZFS experts (namely Linda) on the list. 
I'm sliiightly worried about what software might now not be able to access 
the pool - I'd like to assume this means older versions of ZFS, and not 
anything else. But I'm not sure. The manpage for zpool-features wasn't 
very helpful...