How to do live updating similar to Google Docs?

I want to do something very similar to Google Doc's live updating - where all users can "immediately" see the actions of the other users in the doc.

To achieve this, my ideas so far:

• Continuous AJAX requests being done in the background (this would seem performance-intensive)?
• Surely there's not a way for the server to push notifications to all its clients and update them accordingly?
• AJAX requests every X seconds with a buffer/time-lapse of actions to be accomplished in those X seconds (simulating a real-time effect)?

I would like to know others experience in trying to achieve this effect. What is the best way to do this?

All help is appreciated.

NOTE: I'm not specifically looking for a real-time document editing solution. I'm looking for a solution to the same concept of what Google does with their Docs. I will actually be using that solution in a slightly different manner.

I vote for Long-poll strategy : each client opens a request to the server, but the server never breaks up connections, and just send pieces of java-script from time to time.

Constant AJAX requests would kill your server.