[...] there's no way to schedule appointments with people for whom I only have an email address, no way to get at my calendar data programmatically, to script it, to view it with an RSS feed, to access it via IM or SMS, etc. Thanks to the webcal URI standard people are starting to publish calendars but there's no easy way to maintain these other than exporting an ICS file from your client and copying it to a server every so often.
Go here to see the ideas these guys have to solve this. Here're some screenshots of Novell's Hula WebClient.
Mozilla Foundation is also working on an XUL based Calendaring client targetted at Mozilla based browsers and another stand-alone one called Sunbird for now.
There have been instances when I have missed a meeting because I was not at my desk and missed seeing Outlook's reminders. It would be great to see IM and SMS based querying and notifications implemented, and its something I'd like to hack on.
Here's an example URL of a calendar.
webcal://ical.mac.com/s.armbrust/F1322005.ics. This happens to be the URL of the F1 2005 Calendar.
There's a Perl module - Net::ICal to provide an interface over the iCalendar calendaring and scheduling protocol (iCal is over an year old).