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That said, the use of an m.example.com is becoming very common for major sites. I think that's the approach I would use.
The goal is to keep your content all under one roof. No need to start another site just for mobile if you can use a separate stylesheet served based on the UA. I'd like to think you don't want to fragment the content. People will visit and bookmark the same pages from your site when using a stylesheet to serve mobile content. Things will be a little different if they are visiting and bookmarking m.example.com or example.com/m/. Remember - One Funnel
What does this mean for design, well for me its a single column layout thank you very much.
I checked one of our sites on the mobile checker that you shared and was shocked to see a big fat ZERO.