Do you want to know how many visitors are viewing your site with a "retina" or similar high-density display? Well, do you use Google Analytics?
You do?! Excellent.
The solution is pretty simple. Here's the gist.
You can get to this information in your Google Analytics account through the Custom Variables area. Neat, eh? It may take awhile to start collecting (up to an hour), but it will eventually appear.
Thanks to @chandlervdw for coming up with the problem to solve. Also check out Rob Tarr's article on Google Analytics and the resize event.