This is slightly embarrassing: Instapaper doesn’t show Retina-resolution graphics on the new iPad yet. It's only slightly embarrassing for Apple's culture of product secrecy, which prevents them from releasing tools in time for devs to have gear ready for release.
I'm pretty sure the simulator that devs use supports the Retina iPad, and was available before release date. The tricky bit is that the Retina iPad is higher resolution that most people's desktops, so simulation becomes difficult.
The simulator now scrolls, however, so you could in theory test retina iPad apps full size in portrait mode on an 11" MacBook Air.
My 27" iMac (2650x1440) isn't quite tall enough even in landscape mode, but it's not unusable thanks to the scrolling.