Photoshop CS5 is the first version that allows you to assign a keyboard shortcut to the Foreground Color Picker. Go to Edit>Keyboard Shortcuts and select Tools in the Shortcuts For drop-down menu. Scroll down until you see Foreground Color Picker and click on it. Enter a letter to change the shortcut. (I chose P for Picker, using the logic that I use the Color Picker way more times a day than I do the Pen tool.) If you choose a letter that’s already in use (such as P), you’ll have to click the Accept button to make this change.
---by Dave Cross