But LOOKOUT!! IT'S A FARCE!!
What the headlines should really say is that Office Online comes to the browser ONLY on Operating Systems provided by Micr$oft.
Check it out for yourself. Fire up any GNU/Linux operating system, open Firefox, go to http://home.officelive.com.
What you'll see is this irritating message:

So don't be fooled.
Yes Micr$oft is putting products in the cloud.
But, NO NO NO, they are not making the products be truly Inter operable with a 1/3 of the OS Markey share. Yes, I'm saying that GNU/Linux is #3 of the top 3 Operating Systems out there.
I say hooray for the Internet. Because the Internet uses standards that can be used by a variety of Operating Systems.
Office online DOES NOT use web standards.
It's another dirty trick to hook you into paying the Micr$oft OS tax.
Don't buy it!!!
Keep your sanity, get Google docs or Openoffice.org instead.