I'm writing a break-out clone, and so far it's working well on my PC. But when I broadcast it to Mac/iPad, the ball either moves really, really slowly, or if I increase the force applied to it, travels right through the bricks without bouncing off.

I understand that physics got changed in the update, so is it a case of still having the original player (compiled on my Mac, pre-update) vs my Windows player, that is updated?

It is very likely. The new 1.0.3.7 build uses the new Box2D, whereas the source included in the above build 'might' have the old Box2D code. If you wish to send me the source and project media, I can run your game on the latest iOS build at this end and I'll confirm this. I will be away in San Fran for the IDF Conference Monday to Friday but will be back the following Monday to catch up on emails. Even though the effect of physics is non-deterministic, both Windows and iOS should reproduce gameplay physics similar to each other. If this is not the case, then it is a bug and should be reported in the issues board.

@LeeBamber - I've mailed you the project. Thanks for looking at it.