According to Omar Shahine (and numerous other sources), the “Mail Beta” or Kahuna (to use its’ codename) is being opened up to more users, and you can sign up here

Best estimates are that 200,000 people will get invites to the new system – so get your name down fast!!

As for my invite, well I heard back from Imran Qureshi today about it:

Unfortunately your account was on one of our old clusters so it will take some time to move it to the new ones that are kahuna enabled.
It should be a couple more weeks.
I’m sorry

So it looks like I might have to wait a bit longer…… bummer 🙁