Six months after Win7 hits the shelves on 10/22/2009, Win8 beta 1 will available to all who want it. It will be tested for 24 months, or 2 years, which ever comes first.
Microsoft will be collecting data from all installations via the sendstuff2ms.exe file installed with the original d/l.
Win8 will be 64 bit only. The time frame allows all the "I-don't-want-64-bit" people to rant, rave, fuss, yell, and scream, before updating their hardware.
There will be no beta 2, no RC, and no RTM editions. The original d/l will evolve via updates over the 2 year period, or 24 months, which ever comes first, to the final final final version on 4/22/2012.
On 4/22/2012, testers will be given to option to purchase the OS at a discount equal to 25% per 6 month period tester tested. SO, if a tester tests for the entire 24 months - OS is free.
If the tester does not purchase the OS, well.....let's not get into that right now, it could get messy.