Public beta is released now, the direct link to the downloads are as follows
Win 7 32-bit ISO
Win 7 64-bit ISO

and PS, here's how you can get the official windows 7 beta key

  1. Visit TechNet Downloads, sign in at the top right.

  2. After the page has loaded, copy and paste the following url
    https://www.microsoft.com/betaexperience/scripts/gcs.aspx?Product=tn-win7-64-ww&LCID=1033

  3. This will give you a product key for 64-bit, it works for the 32-bit OS and vice versa. The server is overloaded, you will need to refresh the page a lot of times before you get the key. While stock list thou, there is only 2.5 million keys to cycle around. Good luck