This is going to sound backwards, but will it be able to pull the ICS file from remote locations, say, for instance, Google Calendar?
The reason is that I have Thunderbird setup to work with Google Calendar, I make my changes within Thunderbird. Everything else then, like my cell phone, hooks into Google Calendar to get updates and syncs, etc. Typically, its a one-way ordeal for anything OTHER THAN manual modification on Google Calendar, or Thunderbird. I don't want to speak for anyone else, but for MY purposes, one-way integration would be perfect.