Is there a tool that will do this? I want to run the simulator, and then I can move the mouse at some point and tell me what the (x, y) coordinates are. Of course, there is a simple tool that does this.
In Developer Tools → Applications → Graphics Tools, there is a program called Pixie. He will do what you want. In the settings, you can configure it so that the drag-and-drop parameter counts pixels. You can also set it to just show pixel coordinates and do the math yourself.
I just use the built-in screenshot from OS X. Just Command + Shift + 4, and when you drag it, you see the snap sizes that you would take. Hit escape to remove it. It works great.
I used Iconfactory xScope for this before. If you create rulers the size of which is displayed in the simulator, you can get the X and Y coordinates of the mouse pointer when moving around the simulator screen. However, the correct rulers with the edge of the simulator screen can be a little complicated for applications with dark backgrounds.