UPDATE: Fixed in OS X Mountain Lion (10.8).
Hey, this may be an educational post for some. When you press "Cmd+Shift+4", Mac OS X will enter into a screen grab mode, where you can select a rectangular area of the screen. The cursor changes to a crosshair and then you click and hold to begin selecting, drag the cursor to select the area you want to capture and finally release the mouse/trackpad button to capture it. The resulting image is saved to your desktop.
When you first hit "Cmd+Shift+4", the cursor shows a pair of numbers which are screen coordinates of the cursor's position. The top one is the X coordinate in pixels, relative to the left of the screen. The bottom one is the Y coordinate, relative to the top of the screen.
Then, while you are holding down the button, the cursor shows a different pair of numbers. These are the size of the region you are selecting, in pixels. The top one is the width of the region and the bottom one is the height. But, sometimes those numbers change back to the screen coordinates of the cursor. Therein lies the usability problem. Observe the behavior:
Now if you're trying to grab an area of the screen of an exact size, like 100x100 or something, having the size of the region toggle like this is extremely annoying. Oh, and I have further discovered that this seems to only be happening with the MacBook trackpad. This doesn't seem to happen when selecting the area with a mouse.
This is something I fight with for each screen shot I take for this blog, so keep that in mind, won't you?