I noticed this last week but forgot to blog about it. If you do a google search and the result is an Open XML file (.docx; .xlsx; .pptx) they give you the ability to view them in the browser using their own rendering technology. They support all three formats, and the results are pretty rich (surprisingly richer than what they provide for ODF files).
Take a look for yourself. I did a search for .xlsx files:
If you click on the "View as HTML" link, they transform it into HTML and give you a rich view:
And if you open it in Excel, you'll see that the preview was actually pretty accurate: