Okay, so the iCal stuff from Apple is pretty cool. But how is this app going to store and exchange calandering info? I mean, really, the question is "what XML format is Apple going to choose?"
A while ago I was exploring the calendar stuff out on the internet for EventEngine and I discovered there really aren't any standard Calendaring specs in XML.
The iCalendar spec is a version of the vCard stuff - a binary file - and then there is, confusingly enough, the vCal spec which is another older spec for general calendaring. The first has been updated to the xCal draft spec but who's using it? A Google search doesn't bring up much.
You can see the age of the vCal stuff just by checking out the valid EMAIL property values: AOL, AppleLink, ATTMail, CIS, eWorld, INTERNET, IBMMail, MCIMail, POWERSHARE, PRODIGY, TLX, X400. Nice... eWorld?
The SyncML stuff that Steve mentioned actually uses the vCal spec inside of it. But SyncML isn't open and covered by a bunch of patents by IBM and Starfish (go Philippe!) so it's not like a general use thing.
The reason I'm analyzing all this is "thinking out loud" about how to create a iCal-compatible web app. I noticed the other day that Adam Curry has his public calendar posted on the his web page. It looks like some Userland app that uses RSS as the calendaring file format. If that drop-downlist was a set of check-marks instead, it would be very similar to the iCal idea... (well, a web-based, not as smooth version, but the same general idea).
The idea is solid - sharing calendars should be as easy as sharing your BlogRoll via .opml or your recent posts via RSS. It would be great to know what Apple was doing and have an app ready for when it was launched... ;-)