[Joel]

This is actually something that's going on at the OS level in order to support HiDPI mode in Mac OS X 10.7 and later. From the Developer Documentation:

"When it loads an image, an NSImage object automatically sets the size and scale properties to appropriate values based on the suffix of the image file. For standard resolution images, it sets the scale property to 1.0 and sets the size of the image to the image’s pixel dimensions. For images with the @2x suffix in the filename, it sets the scale property to 2.0 and halves the width and height values to compensate for the scale factor. These halved values correlate correctly to the point-based dimensions you need to use in the logical coordinate space to render the image."

We've put in a fix to handle this more reasonably in OmniGraffle 5.4, until we can figure out whether or not we want to support the @2x suffix conventions, and where. Amusingly enough, NSImage supports @4x, @5x, etc. as well.