I already use Google Docs for collaborating simultaneously on taking notes or creating draft documents for university courses and its great. You can write on the same document on the same time with your colleagues and you'll see (nearly real-time) your colleagues contributions and editing. But those things shown at the preview video really beats all. As they mentioned, it perfectly merges instant messaging and email writing or collaborative document building. And as usual, a really neat UI. You have to take a look at it, it's great.
Another great feature is the way of being able to add extensions that can be seemlessly integrated in the main UI.
Take a look at the video, not bad...and as also Manfred mentioned, watch out when the automatic translation robot comes to work ;)
Here's a longer story by Lars Rasmussen, one of the lead engineers at Google Wave which also built Google Maps.