My way around this was to add an Exchange account to iCal. So now I get my calendar in both places. The only thing that ***** about that is invites. I get double invites. But as I stated in an earlier post, I'm using BusySync to push my Exchange calendar to my google account. So I just ignor the Exchange account when I'm in iCal and just look at the sync'd Gmail calendar. That was my way around it. Also, I've tried this on both Snow Leopard AND Lion. So it's not just a mountain lion thing. Funny thing happened after all this messing around. I shut my system down and rebooted everything. The next time i opened iCal my sync'd outlook calendar was working again.
I'm pretty sure it was the microsoft update that did it. But apparently a reboot did something. You might have better luck posting this on a Microsoft forum. Or you can give my BusySync solution a try... Good Luck!