The GData API that Google promised was released last night -- and it's quite interesting. It almost resembles what Microsoft tried to do by extending RSS2.0 with SSE. I must admit though, I am not terribly familiar with SSE and would like to hear from you if you have more information about the similarities and differences between it and GData.
Google's solution makes use of both RSS, Atom (both the syndication format and publication protocol) and a model for queries/results. I haven't used the API (yet), but it looks like it will be well recieved by the development community.
Google Calendar uses the GData interface and thus makes it easy for programmers to integrate with it. With the ease of integration and considering the popularity of Outlook, I wonder how long it will take for someone to write a plugin for it that seamlessly integrates with Google Calendar? My guess is, not long.
If you are interested in the Google Calendar Data API, you should subscribe to this group -- it seems like it will be a great place to get more information as people start using the API.