I wonder if I'm missing something, because I would expect this to be a common request:
It would be great to have the option to create a @2x version of the same file that takes advantage of the retina display. Or alternatively, an update to the ParticleEmitter class to take care of this in code.
In the game engine from the book, I'm setting the scale of the glView to make use of the retina display. I've done some modifications to the Image and BitmapFont classes to make them work well at both the regular resolution and the retina display resolution.. But I'm not sure how to modify the ParticleEmitter class to make it work well at twice the resolution.