A quick search didn't show a thread on this yet.

The beta details have come out - http://www.bungie.net/7_The-Destiny-Bet ... ?aid=11643" onclick="window.open(this.href);return false;
Starts 17/07 on PS, 23/07 on Xbox (both at 10AM PDT / 6PM UK) and they say it'll end on 26/07, but they did extend the alpha so maybe not...
The alpha was fun, and if you want a beta code you just need to pre-order at a participating retailer (you should still be able to cancel after getting your code, if you choose to).