Google Calendar isn't just a personal calendar -- as many have already discovered. It is a powerful tool that can be used for so many other purposes. Anything from healthcare to television listings can be enhanced by using this service -- both of which we know Google has an interest in.
An article by Clinical Cases and Images walks us through an example of how doctors could use Google Calendar to make their schedule publicly available to patients. For simple scheduling purposes like this, Google Calendar is a great way for businesses to provide useful information to the people who need it.
When Google creates their rumored DVR, wouldn't it be cool if you could check television listings on your personal calendar and flag programs to record automatically? I think this has a real possibility of happening.
These are interesting ideas, but how will this service gain enough popularity to make it as ubiquitous as their search engine? The answer would be through tight knit communities with influential leaders -- like church congregations or even schools. For example, church members could easily subscribe to an events calendar, where school teachers could announce things like exams and assignment due dates.
The bottom line is that Google Calendar has potential to be the calendar for the world, given the company's large user base and it's simple calendar sharing capabilities. If enough people adopt this tool (which I think is very possible), Google would quickly become the leader in personal scheduling -- forcing others with similar products to shift gears and enter chase mode.