Add the following as a Build Phase Run Script. Borrowed from GrowlMail, except quotes were added around the arguments to install_name_tool
since our project has a space in the name.
install_name_tool -change "@executable_path/../Frameworks/Growl.framework/Versions/A/Growl" "@loader_path/../Frameworks/Growl.framework/Versions/A/Growl" "$BUILT_PRODUCTS_DIR/$EXECUTABLE_PATH"
install_name_tool -id "@loader_path/../Frameworks/Growl.framework/Versions/A/Growl" "$BUILT_PRODUCTS_DIR/$FULL_PRODUCT_NAME/Contents/Frameworks/Growl.framework/Growl"
Here's a (lengthy) explanation of why this is necessary.