Well, adding support for Retina displays is definitely an iOS-specific thing (at least right now).
Not "right now". As long as you intend to support "retina displays", it will always be an Apple-specific thing.
If you want to support something as general as high-DPI devices, you know, something concrete and not an Apple marketing-term, there are certainly lots of those being made and sold as we speak. They run Android and are not made by Apple.
You might want to consider supporting those as well.
I downvoted you because your post seemed unnecessarily pedantic and condescending to me. I think it was pretty clear that GP was casually (or accidentally) using "retina display" as a generic term for "high DPI display".
Still: targetting "retine display devices" is wrong for a multitude of reasons. You don't want your website to be tailored for a device which has 4x the number of pixels of another very specific device made by one vendor only. That is not the problem you are trying to solve.
The problem you are trying to solve is creating a DPI-independent site layout which exploits the resolution and estate available, while degrading gracefully on lower class devices.
If your solution to that problem is making your website hardcoded to fit two devices made by one vendor, as long as they remain true to leaving everything pixel perfect as they are now, for all eternity, needless to say, it's not a good nor elegant solution.
It's the "This website was made for MSIE" of olden days all over again, only this time for Apple-devices. And thats just not going to cut it.
The server should not have to decide which image to send, based on whatever it can gather of a combination of various factors, including device display resolution capability (double-density vs regular pixel mapping) and connection quality (wireless vs broadband) or whatever else comes into play. The server should be advertising the capability of high res images and the user agent should be requesting them if it wants.
That's a great idea, but that's not currently how it works. All of the high-res website image stuff that I've seen all uses client-side JavaScript to fetch the 2x images.
Not "right now". As long as you intend to support "retina displays", it will always be an Apple-specific thing.
If you want to support something as general as high-DPI devices, you know, something concrete and not an Apple marketing-term, there are certainly lots of those being made and sold as we speak. They run Android and are not made by Apple.
You might want to consider supporting those as well.