Charlene Li of Forrester research has a blog post entitled Google Calendar creates a platform for "time" applications where she writes

Having trialed a half dozen of them (including Airset, CalendarHub, 30Boxes, Planzo, and SpongeCell), Google Calendar is truly a best of breed in terms of ease of use and functionality. Here’s a quick overview of what’s different about the new product:

-          Manage multiple calendars. ....

-          Easy to use. ....

-          Sharing. ....

-          Open platform. I think this is the most interesting aspect of Google's calendar. The iCal standard along with RSS means that I will be able to synch my work calendar with my Google calendar. Although tie-ins with programs like Outlook aren’t yet available, Carl Sjogreen, Google Calendar’s product manager, said that such functionality will be coming "soon". Google is also partnering with Trumba to enabled "one-click" addition of events to your calendar (Trumba already works with calendar products from Yahoo!, Outlook, MSN Hotmail, and Apple). Also promised are synching capabilities to mobile phones. Carl also said that an API was in the works, which would enable developers to create new products on top of Google Calendar.

I've always thought that Web-based calendaring and event-based products haven't hit the sweet spot with end users because they are too much work to use for little benefit. The reason I use calendaring software at work is mainly to manage meetings. If I didn't have to attend meetings I'd never use the calendaring functionality of Outlook. In my personal life, the only times calendaring software would have been useful is integrating invitation services like eVite into my calendars at work and/or at home (I use both Yahoo! Mail and Windows Live Mail).  However either eVite doesn't provide this functionality or it's unintuitive since I've never discovered it. So web-based calendaring software has been pretty much a bust for me. AJAXifying it doesn't change this in any way. 

On the other hand, I could probably build the integration I want between my work calendar and my eVite calendar if they had an API [and I was invited to enough parties to make this a worthy excercise]. It seems there is now an awareness of this in the industry at the big three (Google, Yahoo and Microsoft) which is going to turn online calendaring into an interesting space over the next few months. Google Calendar is a step in the right direction by providing RSS feeds and announcing a forthcoming API. Yahoo! is already thinking about the same thing and also announced an upcoming Calendar API last month. As for Windows Live, our CTO has been talking to folks at work about using RSS+SSE as a way to share events and I'm sure they are paying attention [or at least will now that both Yahoo! and Google have thrown down].

With the increased use of RSS by Web-based calendaring applications perhaps it is time for RSS readers to also become more calendar aware?