As announced in Ryan Spain's latest article, the Wide Beta for the new Magic Online started today, September 4, at 10:00 AM PDT. If you participated in the previous Sneak Peek, you can just launch that version of Magic Online and it will update automatically. If you have not tried the new game client yet, just visit the download page to get started.
Since the Sneak Peek, we have made several key changes with more on the way (check out Ryan's article for the full scoop). This is your chance to check out the progress we have made and give us your feedback. Use the Magic Onlineforums to discuss your favorite feature or make suggestions about what could be better. In addition, we will offer surveys as the Wide Beta progresses which will let you provide feedback directly to the development team.
Please remember that actions players perform during the Wide Beta will affect your actual Magic Online accounts. The interface may be different, but the effects are still permanent as if you were using the current MTGO client, so don't get crazy and start trading all your Worldfires for Centaur Coursers.