[ikemstar]

Hi Skwirl,
My problem is solved, so I am posting my solution here just for future reference - it won't work for most others. Safari on Mountain Lion has huge issues and under certain conditions freezes my entire computer. It forced me to reboot; when I tried to open my OmniPlan file, it point blank refused to do so, under any conditions. Installing a new copy of the application wouldn't do it. Deleting all support files wouldn't either. I had a copy of the file mirrored onto a work machine at another location - and it opened there! Saving as another file name, I was then able to open it on a fresh install of OmniPlan. Beats me why these issues occurred, but that's how it worked for me.