One trick to do that (for iCal, Address Book, Keychain, etc.) is to move their data folders to within the Dropbox folder, then put a symlink pointing there at their original locations. There are various guides like
this one to show you how. You'll want to combine those two commands into one to minimize the time the folder is unavailable, though, like so:
Code:
(cd ~ && \
mv "Library/Application Support/AddressBook" Dropbox/ && \
ln -s ~/Dropbox/AddressBook ~/Library/Application\ Support/AddressBook)