Mary Jo Foley writes about it.

I recently saw a WSJ article about Ajax which speculated that someone would soon write an Office competitor in Ajax. 

Uh...where do I start? 

Ajax is fundamentally limited by its components:  HTML and Javascript (I'll give XML the benefit of the doubt).  Or more accurately, its fundamentally limited by the browser in which those technologies are hosted.  To take the next step to building something like Office, at the minimum you need better graphics.  Going back to the server to render your bitmaps isn't going to provide the Rich Client experience.

Go take a look at the much vaunted Google Maps:  pretty impressive...considering it's run in a web browser.  But compared to a full client-side mapping program, like MapPoint, or Google's Keyhole, Google Maps looks kinda sick.  That's the key with Ajax:  it's pretty impressive...considering it's run in a web browser.

It's like the story of the Dog who talked.  It wasn't that he talked so well, but that he talked at all.