The iOS development environment facilitates image content loading for retina and non-retina devices with a clever naming scheme. If an iOS app is running on a retina device, and is instructed to load an image named foo.png, it will actually attempt to load the email@example.com file if it exists. By providing both foo.png and firstname.lastname@example.org images, both classes of devices are supported.
This convenience offered by UIKit goes one step further: if you do not provide the regular resolution version foo.png but only the retina version email@example.com then non-retina devices will load the high res version and automatically downscale a factor 2 so that it can be used. If you are tempted by just providing retina versions of your images, and skip the regular versions, like I was, you are selling your app short. It turns out that UIKit does a horrible job at downscaling, and the image will look considerably worse than a version that you pre-scaled yourself.
To illustrate the effect, see below how the original is downscaled by UIKit, and how it is downsampled by my authoring tool inkscape. A big difference, I would say.
The bottom line: if you care for your iPad2 and iPhone3GS Users, don't skimp on images. Provide both the foo.png and the firstname.lastname@example.org files.
The retina version of the graphic, viewed at 1:1 zoom.
The image automatically downscaled by UIKit from the retina version, viewed at 2:1 zoom.
The image, exported at non-retina resolution by inkscape, viewed at 2:1 zoom.