We have one user, me, for whom our Mac users cannot seem to access the calender from iCal. This was driving me crazy for a while, but the problem was really really stupidly simple. I had my work day set from 6a to 8a rather than from 6a to 8p, so most of the day was 'unavailable' because it was outside of my work day and iCal, unlike Outlook on the Mac, doesn't show anything that is scheduled during unavailable time so it appeared that my calendar was just not visible. Dumb Dumb Dumb.
Exchange Server hosted with Intermedia
PC users on windows 7/Outlook 2010
Mac users on Lion/Outlook 2011 and iCAL