Sorry, late to the party here! In OmniFocus 1.9.1, we added a hidden default for this. You can install the 1.9.1 Sneaky Peek builds
here.
To restore the old behavior (double-click to focus), open up Terminal and enter the following command:
defaults write com.omnigroup.OmniFocus DoubleClickToFocusRequiredModifierKeyFlags -int 0
Hope this helps!