Count me in on this one as well. It would really help me with getting lines straight and locations right.
I'm not a pro at drawing, so I might actually need this.
What I do now is selecting the select tool, then the snap tool, then clicking on the centre of the image (which the selection tool shows, together with a grid) and then make a snap point so you can draw straight lines up and to the side. From there you can make your own grid and make it transparent enough to draw under. It works but it's a lot of work to get right.