For more details of the "Why" decoupled question, check out our blog post 'Why We're Decoupling Drupal':
http://www.pixotech.com/decoupling-drupal/

For more of the "How" to decouple topics, watch for updated how-to's on the Outpost site:
http://www.getoutpost.org/

Outpost is an open source framework for decoupled websites. Also, the "How" question really has multiple answers, and two good case studies are covered in-depth in the Webinar video Emily linked to above.