Business Division President Stephen Elop said in an October interview that a technology preview of the browser-based Microsoft Office for Web, codenamed Office 14 would come this year, followed by a beta in 2009. However, it turns out Microsoft is using a rather tortured interpretation of the term "technology preview."
It's currently being used by fewer than 1,000 Microsoft employees, as part of a test that started last month and is slated to go through February. Consumers won't be able to try a test version of the products until sometime next year. Microsoft isn't saying when, but I'd say you'll have plenty of time to try the Windows 7 beta before you have to worry about testing the Office Web apps.
Ultimately, Microsoft is planning the release of the Office Web Applications in conjunction with the next wave of Office product, code-named Office 14. Microsoft has not offered a release date for the desktop version of Office 14.