I think the simplest way is just going to be to use the iPhone's built in mechanisms for managing which images to use for the retina display and so forth.
I stumbled across an article here:
2010 August | Sparrow | Hatch your App! (scroll down to the part about "creating the graphics") in which the mentioned framework provides a script that will automatically create all of the variations for the graphics and put the @2x suffix on the filename and so forth. It may be worth downloading those utilities even if you dont find the framework useful.
