I'm more than a little skeptical of this positioning, even though a Microsoft developer characterized Excel Services as a product that "will probably be competing with the likes of Google Spreadsheets, DabbleDB, Zoho and JotSpot Tracker." (Developer Tod Hilton went back and edited that part of his post out of his blog. But TechCrunch grabbed it first.)
Excel Services isn't new. It's part of Office 2007, specifically, of SharePoint Server 2007. Excel Services is not a Web-based version of Excel. It does allow users to access Excel data via a Web-based interface. But Excel Services lets users get at Excel data stored in Excel Server (which is based on SharePoint) and share that data among those with approved access to it.
Office 2007 includes a number of other "services" that are akin to Excel services. There's a Forms service that gives users access to InfoPath documents stored on a server. There are PowerPoint libraries that give SharePoint users shared access to PowerPoint slides. And I've been hearing rumors of Access database services coming as part of the Office 14 wave. These services are all contingent on Microsoft SharePoint Server.
Is Microsoft going to bundle these up and position the collection as a Google Apps or Zoho competitor? I bet not.
Microsoft doesn't want to give its blessing or backing to a completely Web-based office suite. Microsoft execs want "software plus a service" -- not "software as a service" to win.
If and when Microsoft does concede that some consumers and small business users want Web-based productivity apps, they're far more likely to throw a Web-based version of Microsoft Works over the wall than a Web-based version of Office, I'd predict.