I reinstalled PHP iCalendar to test a theory.
I uploaded my .ics file via the admin interface in PHP iCalendar, and made sure it was chmodded 0755. I could view the calendar just fine, although some errors about template files printed at the top of the page.
So I decided to attempt to publish the .ics file with iCal using WebDAV. Although it published, it's now chmodded 0640 ... and since I'm using WebDAV on DreamHost, I do not have the ability to change the permission of the file.
From this, I have to assume that iCal is changing the permissions of the file when it publishes it via WebDAV, and PHP iCalendar simply will not display any .ics file with those permissions set.
Any ideas on what I can try next? Keep in mind that this was working perfectly for me for a long time in Panther. Only after I upgraded to Tiger did this break.