The talked-to-death treadmill deck project is finished. I'm currently tuning the belt on the treadmill, which might take a while, but the actual deck appears to be rock solid.
Simple design, a composite deck made from 2 4'x10"x3/4" and 1 7" wide poplar boards, with 2 cross braces added to the underside to take the load off the glued joints and stiffen the deck overall.
Once I get the belt tuned I'll put it through it's paces and see how it holds up to a workout, but if the small tests I did tuning the belt are any indication it's a heck of a lot stronger than the MDF board.
Only remaining task for the deck is to apply a durable high-gloss finish to reduce surface friction. I'm thinking paint, as wax would wear out too fast and need re-application.