Dropbox is great for testing retina iPhone mockups. Your design files are twice the size of the final product and it’s important to view them within the context of the device.
Create an alias/shortcut to your Dropbox testing folder on your desktop. Grab a screenshot the 960 x 640 design and throw it in the Dropbox alias/shortcut folder. Open it it up through Dropbox on your iPhone and view in @2x retina glory.
I find the workflow between screenshots and the alias/shortcut folder quick enough to allow rapid iteration.