What an amazing program. I was just telling a friend of mine tonight that I was going to write Blizzard a letter, because I am an add-on author who was at the Vanilla midnight release and I have purchased every expansion... But the only invite I was able to get for testing was TBC.
I haven't been active on the forums here, so its my own fault that I was not given an invite for this beta. But this is exactly what I was envisioning as a solution to the problems of authors getting no time to develop and test before release.
Great job, kudos to the WI people and Blizzard for making this a possibility.