I just switched to Tiger, thus moving to iCal 2.0. While most of my
subscribed calendars are still working nicely, at least one got
silently dropped with no comment whatsoever.
In addition to the trouble of having to find the corresponding URL
again, I now find that I cannot subscribe again, getting a message that
"Data downloaded from ... is not valid". When I download it first, I
can import it as a vCal calendar, whereas importing it as an iCal
calendar yields "This calendar file is unreadable".
With earlier versions of iCal subscribing to that same calendar works