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Open up the Script Editor application and paste in the following:

tell application "Address Book"
end tell
Save it in your Library/Scripts/Applications/OmniFocus folder (may have to create the OmniFocus folder in Library/Scripts/Applications if you haven't previously installed any Applescripts for OmniFocus). Now go to OmniFocus, select View->Customize Toolbar and drag the icon for Script: Address Book (or whatever you chose to call it) to the desired spot on the toolbar. Now you have your very own button to fire up the Address Book and bring it forward. Building a similar button for iCal is left as an exercise for the reader, as is customizing the toolbar icon.