hm, now I've got myself thinking...
Started with the concept of using email as a notification method, since you can opt to CC to say work and home addresses, so get reminders anywhere you happen to be. That would imply an online app, like any online calendar, so I thought - nah, I hate putting personal information on an online web site. I don't even use webmail services, I want my stuff to be MY stuff.. etc.
Then I thought hang on, what about a desktop app which just sits in the background and communicates with the online calendar app? The database resides on your PC, where the small "engine" app serves as middle-man between web site requests and the local database.
So basically you have the convenience of using a web site to view and update your calendar.... but your info always resides on your home PC. The local running app would have to open a port and be a server, like any normal P2P app does.
The web site itself is merely the "thin client" for the app running on your PC.
hmmmm.....