"FuseCal lets you sync any calendar to any calendar, while also providing filters that let you limit the types of events that are included in the sync."I haven't tried it since earlier days when it wasn't quite ready for prime time, but it's an innovative quasi-screen scraper/Web page calendar detector that looks promising. I'll be trying it out with Mozilla Sunbird in a few days to see if they play well together.
UPDATE: My quick check of FuseCal reveals that any FuseCal can import an .ics file to any compatible calendar, but only supports a limited set of calendars for subscribing to iCal feeds. Here's hoping they add Sunbird subscription support soon.