> You could even TimeMachine-without-additional-drives... I mean, why should my snapshot system and my backup system be one? How about letting me use TimeMachine locally, and if I plug up an additional drive, TimeMachine asks me if I want to backup to it as well?

Unless something changes between now and RTM, Lion will provide that feature out of the box:

http://www.9to5mac.com/54117/time-machine-can-be-used-withou...

And that's about the only feature that a typical Mac user would benefit from with ZFS, without some kind of "prosumer" or "enterprise-y" external storage device.