Originally posted by JohnMc I can't recall a version of PS not having a grid guide in the crop tool.
I think what the OP is referring to is the inclusion of the "rule of thirds" guidelines that SilkyPix originally included in it's cropping tool.
If, for example, one wanted a nicely framed 16:9 image, by choosing the appropriate ratio, the lines would appear within the cropping frame "auto-magically" and would move appropriately as the cropping frame is re-scaled.
So, with a combination of moving the crop within the image and resizing it to suit, the final composition could be arranged to conform to the "rule of thirds".
It's a concept that works in any aspect ratio and can help the "less artistic" of us produce aesthetically pleasing images
It's a feature I use frequently in IrfanView and would welcome in DCU if it was made available