I've been using the Truncate Silence function in Audacity to do this, since I was doing some editing of podcasts before listening, but didn't start speeding things up until I started using Podkicker with the Prestissimo plug-in.  Accelerated listening has really helped me catch up with a 3-month backlog of podcasts. :)
Anyway, to the question... I played around with the sox silence command for a while, trying it out, and got it to work as you said, but what I could not do was to leave the leading silence alone.  Man pages, help pages, reference pages, etc all assume you want to get rid of the leading silence, and show how to do that, but don't demonstrate how to skip the leading silence.  Nothing I tried worked.  Is that something you've tried to do?