I'm not aware of anything in OmniPlan itself to do that, only the other direction (export OmniPlan tasks to iCal calendars or to-do lists.)
The iCal .ics format is not all that complicated. If you've entered your tasks as iCal entries that don't span multiple days, and don't have all day entries, it seems like it might not be too difficult to cobble something up that ran over an exported file from iCal and produced a list of task names with duration and start date/time, then wrote that out as a .CSV file that you could import. Of course, OmniPlan won't know any of the details of the structure, resource allocations, etc. but you'll at least have the blocks in there to start tinkering. You won't really get the full benefit of OmniPlan until you start extending your plan beyond what is in iCal and/or edit the stuff you've imported because it will all be entered as exact scheduling and OmniPlan won't move any of it around.
Any idea how many iCal entries you're talking about? If you go to File->Export in iCal after selecting the calendar that contains all of this stuff, export a copy to your desktop, open it up in an editor that will let you count matches and see how many matches you get for BEGIN:VEVENT that should give you a pretty good idea.