Seems to work for me with OmniPlan 2.1. Here's what I did:
I went to
http://calendarlabs.com/ical-calendar-holidays.php and found a calendar full of holidays: webcal://www.calendarlabs.com/templates/ical/US-Holidays.ics
I then went to my OmniPlan document, went to the calendar view, clicked on Extra & Off Hours, then did Project->Configure Publishing & Subscriptions...
I clicked on the + button and chose Subscribe offtime/overtime from web calendar, pasted the URL into the URL box, and was rewarded with some red numbers on the calendar at lower left, and big red blocks on the time view at right.
