You could use CSS media selectors to detect retina displays. Using the display property, you can switch the images on and off depending on the device.
I don't have an Apple retina device at hand to test, but I think something like this should work:
<svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="640" height="480">
<style type="text/css">
@media all {
#test_2x_png {display:none}
}
@media (-webkit-min-device-pixel-ratio: 2) {
#test_png {display:none;}
#test_2x_png {display:inline;}
}
</style>
<defs>
<pattern id="test" patternUnits="userSpaceOnUse"
x="0" y="0" width="130" height="100"
viewBox="0 0 130 100" >
<image xlink:href="test_2x.png" height="100" width="130" y="0" x="0" id="test_2x_png"/>
<image xlink:href="test.png" height="100" width="130" y="0" x="0" id="test_png"/>
</pattern>
</defs>
<rect id="rectangle" stroke="rgb(29, 29, 255)" fill="url(#test)" x="50" y="47" width="320" height="240" />
</svg>