One of the more interesting notions I have heard at Drupalcon has been with the idea of targeting a device first as opposed to a full website.
A particular figure shown was that by 2013, there will be 1.83 billion mobile browser devices vs 1.78 billion desktop browsers, and the idea that web site planning should focus on this segment of browser first as the segment grows and overtakes all others.
Now this idea falls outside of practices specific to Drupal, and I think it has yet to be fully evaluated, but will soon be extremely important. It is a change that will cascade throughout every aspect of website creation: site planning, design, development, and (crucially) client communication. Complicating this notion further is the question of standards supported across devices.
While the bar has been lowered for cross browser testing compatibility (and “lower” still involves running an entire OS on your machine, thank you Internet Explorer), anyone who has had to test across mobile devices is still at the mercy of acquiring an ever-increasing spectrum of mobile devices.