iPad frequently "forgets" the session cookie for my website. On the first hit of the day, it seems to load some or all of the images before setting the cookie, thus loading the wrong images from the server. Subsequent pages will do HiDPI correctly, but I still need to hit the Reload button to get rid of any images cached by that very first hit.
So, the server-side strategy seems to be flawed. When I have time, I will do some testing with the client-side version to see if that works better.