I ran across a link for this product today and decided to check it out. Go to www.firebase.com and check out the examples and tutorial. The product is currently pre-release. You can sign up for an account if you want to play with their API.
You can attach JS callbacks to the data so that your app can be notified when that data has been changed by another client. That's really it's claim to fame. One drawback that I see right now is that there is no client authentication and all of the data is wide open. The examples are in JS so the code and paths are all open. Anyone taking a look at your JS can play with all of your data.
Firebase definately looks like an interesting product and and certainly fills an interesting niche. Take a look and let me know what you think!