In PhotoShop CS 4 & 5.5, with a NTSC D1, 720x 486 document, a Pixel Aspect Ratio of D1.DV NTSC (0.91), and ellipse options set to unconstrained, with Aspect Ratio Correction Turned on, to draw a perfect Circle, you could hold Shift-option-cmd and you'd get a perfect Circle.
In PhotoShop CS 6, the circle is streched to the top and bottom when I do a shift-Drag to draw a circle in a non-square Pixel document with Aspect Ratio Correction turned off.
So, at this point, In a Non-Square Pixel document, There's no way to draw a perfect circle.
What's changed between 5.5 and 6?? Is there a Preference somewhere that I've missed???
Looking into this a little more, what I think is going on is that in previous versions, Shift dragging compensated for the non-square pixels, so that drawn items show up correctly. If you look at the Info Box in 5.5 when shift drawing a circle, the height is greater then the width, when working in a non-square document.
In 6.0, when you do the same thing, the height and width are the same. So, no compensation is being done for the non-square pixels.
In 5.5, if you place a circle by entering the dimensions in the ellipse option, say 400, by 400, with PAR turned on, the image is squeezed side to side.
So, In 5.5 Shift Compansated for nonSquare pixels. In CS6, it does not.