It is. In a presentation at Superconf, one of the Github designers (whose name escapes me) said that they serve some images at high resolution and resize in the HTML tags for the benefit of iPhones 4 and other high-dpi devices.
What browser/OS are you in? It looks pretty reasonable in Chrome on OSX; we spent a decent amount of time trying to get it as close as we could. Some browsers do weird math on resizing.
