There is now word that this app will be integrated into Google's Gmail service "soon".
François Beaufort is typically well-informed, so I would put this information closer to an actual product than pure rumor. Beaufort said that Google's Chrome developers are working on an integration of the Document viewing app directly into Gmail. If you are not using Microsoft Office on your computer, a simple and lean implementation of this app would surely make the life of Gmail users much easier.
If Beaufort is right, Google could simply be leveraging an existing extension for Chrome that can be tied to the Gmail service as a another example how Google is suggesting its users to switch to Chrome.
In its current version, the Document viewer supports the formats doc, .docx, .xls, .xlsx, .ppt, and .pptx.