First, launch Script Editor paste the code into a new script, and save it to ~/Library/Address Book Plug-Ins/Address Book Driving Directions.scpt. Then when you launch Address Book, there'll be a new menu item when you click on an address caption.
If your card has more than one address, you have to set one as the default starting point by selecting the 'Driving Directions From' menu item. This will open a browser window at MapQuest's driving directions form with the starting address pre-filled. It also sets that address as the default value for driving directions from other addresses (its ID is saved in Address Book's defaults).
Then when you click on an address in any other card, the menu option is called 'Driving Directions To,' and it uses the address you selected above as the starting point. Feel free to improve the script, but please post any bug fixes in follow-ups to this hint. Enjoy!
[robg adds: I tested this one, and it works as expected...]

