As ZDNet reported, we've known that Microsoft was working on both ad-supported and subscription-based Web application versions of Office 14 since PDC 2008.

“Microsoft is saying it will deliver Office Web applications ‘through Office Live.’  There will be both ad-funded and paid-subscription versions of these Web apps. For business users, Office Web applications will be sold as a hosted subscription service and through volume-licensing agreements. For consumers, Office Web Applications will be ad-funded and free.” You can rest assured that Office 13 client application (Word 14) will not be serving you ads.

Source:→ Ars Technica