It looks like ZFS deduplication have finally arrived! Jeff have made the following putback that will be part of build 128:
PSARC 2009/571 ZFS Deduplication Properties
6677093 zfs should have dedup capability
Have a closer look here: http://hg.genunix.org/onnv-gate.hg/rev/e2081f502306
I post more details when I've had some time to look at the change.
Update: No need for me to blog about it, Jeff has his own blog with a brand new entry on dedupliation: http://blogs.sun.com/bonwick/en_US/entry/zfs_dedup
Talks I have given
2 weeks ago