Corrected backwards deltaT logic.
I had thought the first time around that motion needed to be adjusted by (target framerate dT / actual dT), so that it would run half as fast at half the framerate.  This is exactly backwards - if the framerate is half as much, everything should actually move twice as fast, since it's only moving half as often.  This fixes really weird (but in hindsight obvious) slowdowns I was seeing on slow machines.  I still need to play with minimum framerate, etc. - on really slow PCs, you can basically float through the Platformer, the jumps are so high.