I've started using TexturePacker from Code'n'Web. There are versions for several platforms (including Mac OS X) and it can be easily integrated into your XCode build toolchain. It can also create standard definition textures and atlases from the high definition (i.e. retina-display) texture, and much more.. It has saved me a lot of time - if you're building a game with lots of sprite atlases, take a look at this tool. Well worth the US$24.95 asking price for the full version.