Hey Simon,
The Australia is lang="en-au"
The UK is lang="en-gb"
The US is lang="en-us"
We've tried to keep these as tight per country as possible so opted not to use the straight 'en'.
In analytics, there has been some reduction is language referrals, mainly "en-gb" falling from the number one language type for the US site, which is a positive. Interstingly enough, once we removed the .co.nz fro mthe Index the .com site remove in to dominate the SERPs for brand and some core-KW searches in Google.co.nz.
Its a little unfortunate as Panda, from my understanding, is keen to spare ccTLDs from any harsh devaluations, but we'll hopefully be able to hit whatever threshold for % of unique content in the near future.
We have review functionality planned for each TLD which should help add value to existing duplicate content. Once this is up and I have some more robust data I'll pull a post together for YouMoz.
Thanks for the feedback!
Kian