2017-04-23 Update: Fixes an issue that could cause the script to fail if certain top-level perspective separators are selected.
2011-07-06 Update: If you downloaded the script before 18 July 2011, there was a bug that could cause an additional hour to be added to the time. That issue is fixed in the current version.
Here’s a script to sum the total time of selected items in OmniFocus. Just select some items, fire it off and see how overcommitted you are.