Search engine here turned this up:
Complements of Kim Mackenzie September 2005
Apple – shift – 3 will capture the entire screen
Apple – shift – 4 will give you a crosshairs, click and drag to select the area you want to capture.
In both cases, the capture will show up as a PDF on your desktop (picture 1, picture 2, etc). Drag and drop the PDF onto Photoshop in your dock to open it.