Found this on Keen and Graev’s forums, through Massively (the original source is on Keen’s post). Listing the beta dates, or at least the potential ones (subject to change from press release date, etc, etc).

They are:

June 19-21

July 3-5, 17-19

August 14-16, 19-23

The June weekend is this coming weekend, featuring Asmodians with a level cap of 10. July 3-5 will feature Asmodians again, with a level increase (I’m guessing through 20). July 17-19 will be focused on the Abyss, both races I assume, and the article mentioned unrestricted flying… I’m not sure if that would be the already open flight available in the Abyss or taking the flight timer away for the weekend — either way, I’m going to be enjoying a ton of PvP. The end of July will see the end of the closed beta weekends, starting from August 14-16 will be the first open beta weekend, with the 19-23 seeing the second open beta. I’m assuming we’ll see the release date in early September (if not the estimated first), and the pre-order head start starting a few days after the end of the second open beta. Most likely running from the last weekend (28th-31st) till launch.

I can’t wait for all of them, should be plenty of fun. For those that are waiting to pre-order or buy the game, the two open beta periods should give you plenty of time to check the game out and get a solid feel for it. I’ll have more Aion posting before them, and posts for each weekend.