I highly recommend using Bjango's awesome Skala Preview app[0] on your Mac and iPhone when designing your interfaces. It essentially mirrors your current Ps canvas to the device and let's you see how it will look in native form. It's invaluable for a multitude of reasons. 1) things really do look much "better" on the retina than for example my thunderbolt or MacBook Air display. 2) a lot of people have a hard time with the initial jolt of designing for 2x the standard dpi. Most of the 1px lines you'll draw for the web need to be 2px in photoshop to appear as 1px on the device.

I have an equally long list of things I would love to share as a long time web developer who moved into iOS and wanted to do things "the right way". This post from SeatGeek and all of the Bjango articles[1] are excellent resources for someone getting started with this

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[0]: http://bjango.com/mac/skalapreview/

[1]: http://bjango.com/articles/