Just to be clear...
"Designing for Flow" doesn't mean creating a challenging user experience. It's simply a recognition that web apps are becoming more and more complex and are helping accomplish tasks that require more time and effort. All sites that seek to help people perform complex tasks are capable of inspiring flow in users.
"Designing for Flow" just allows you to measure your site against four simple rules: does it set clear goals, provide immediate feedback, maximize efficiency and allow for discovery? You can make these assessments on a site-wide level or in reference to a specific task. However, it's not a replacement for proven usability techniques like those found in Don't Make Me Think or Designing Web Usability. "Designing for Flow" is a filter through which to view the design of your website.
Such an important issue. What amazes me often is against how many old (and wrong "everybody does it like that") thoughts one has to go, just to be able to get a step closer to true user friendliness (which, in my opinion, in the end results also into sellers happiness).